This page in a nutshell: Unregistered users can edit articles and participate on talk pages in the same way as registered users. Their input is just as important in building consensus.
Unregistered users are unfairly viewed as disruptive vandals, and some editors routinely revert their additions.
Many users believe that unregistered users' sole contributions to Wikipedia are to cause disruption to articles and that they have fewer rights as editors compared with registered users. Studies in 2004 and 2007 found that although most vandalism (80%) is generated by IP editors, over 80% of edits by unregistered users were not vandalism. As current policy stands, unregistered users have the same rights as registered users to participate in the writing of Wikipedia.
You are an IP too. See here if you don't think so. The only difference between you and an IP contributor is that your IP address is hidden. When you registered for Wikipedia, you hid your IP address behind a user name. While unregistered users were often called anonymous editors, in fact, because your IP address is hidden, it is you who is more anonymous. (Your IP address is still recorded by the software. It is simply not visible to most users.)
Remember this when dealing with unregistered users. They are not a lower category of users. They are not a special subset that we tolerate. They are not locust swarms intent on destroying your article. They are individuals, the same as you. Why does it matter that they have not registered for an account? Just as you deserve to be treated with civility and good faith, the edits of unregistered users deserve civility and good faith from you. As your contributions to talk pages deserve to be heard and counted when forming consensus, so too do the contributions of unregistered users.
Our readers are IPs too
Our readers are IPs too. Virtually none of our readers are registered users. When an unregistered user makes an edit to an article or posts a comment on a talk page, these are the views of one of our readers. That doesn't necessarily mean that their view should be given greater weight. It means that we should not discriminate against their view just because they don't have an account.
Many users believe that policies and guidelines only apply to registered users. Not so. Policies and guidelines affect all users, registered and unregistered, equally.
Comments by unregistered users on talk pages don't count: Yes, they do. The purpose of talk page discussion is to build consensus. Contributions from unregistered users are just as important in determining consensus as contributions from registered users. Unregistered users edit here too. Almost all of our readers are unregistered users. Comment on the contribution, not the contributor. Never disregard a contribution just because it was made by someone who has not registered for an account. Remember, don't be a jerk.
Analysis of 248 edits to English-language Wikipedia articles from 04:43 to 04:46 UTC on 18 Feb 2007 (Source)
Unregistered users are more likely to vandalise articles: This is true; however, the greater proportion of their contributions are non-vandalism edits. In a February 2007 study of 248 edits, 80.2% of vandalism was done by unregistered editors. But 81.9% of edits by unregistered users were not vandalism. Non-vandalism edits by unregistered users accounted for 29.4% of all article edits. Of the article edits, only 6.5% were vandalism by unregistered users; in contrast, unregistered users reverted over a quarter (28.5%) of all vandalism. 91.9% of the edits to Wikipedia articles were constructive and unregistered users accounted for nearly a third of those. Another study carried out by IBM found "no clear connection between anonymity and vandalism"; in addition, the research group found anonymous users provide significant and substantial positive contributions.
Unregistered users are more likely to be sock puppets:Sock puppetry is the use of multiple IDs to evade blocks or bans, or create the appearance of a greater weight of opinion than really exists (see ballot stuffing). That can include the registration of multiple named accounts, logging out of a named account and commenting anonymously or by connecting through multiple anonymous IP addresses. Nevertheless, assume good faith unless you see signs of sock puppetry. Do not assume all IP-editors are sock puppets.
Unregistered users don't know or understand policy: Maybe - and registered users often don't know/understand policy, either. An unregistered user may be a one-off contributor or a first-time editor (it's just more difficult to tell). Bear that in mind and remember: don't be a jerk and don't bite the newcomer.
They should register for an account (e.g. if they want to participate): No. You need to accept their contributions, heed their suggestions and participate in consensus building with them. There is no requirement for anyone to register for an account before they can participate in the building of this encyclopedia. There is always the requirement that you behave.
IP "hopping" is always done to try and deceive: While this is a tactic sometimes used to evade blocks, it is not in and of itself indicative of any intent to deceive and the contributor may not even be aware of it. Depending on where and how a user is contributing, their IP may change, sometimes between nearly every edit. The same thing happens with registered users, it simply doesn't show up the same way because they have an account.
What an unregistered user can't do by themselves (directly)
As a general rule, unregistered users can do everything that registered users can. Unregistered users may edit articles, participate in talk page discussions, contribute to policy proposals and do (almost) everything else that a registered user can do. There are some specific restrictions on what unregistered editors can directly do without the assistance of an admin or a registered-and-autoconfirmed editor.
Directly create articles: Unregistered users may not create articles with a single click. This restriction was placed on unregistered users in response to the Seigenthaler incident. Unregistered users may indirectly create an article. The most common way is to submit an article at Articles for creation, so it can be created by any registered user. Though less common, it is also possible to create content for an article in a sandbox or user-talkpage, and then ask a registered user to create the actual (initially blank) article, after which the information can be copied over. Similarly, they are able to fully participate in deletion discussions, and have been since 2005.
Directly edit semi-protected pages: Some articles (particularly biographies of living persons or seasonal articles, such as Christmas) attract vandalism or persistent breaches of policy from infrequent editors, be they registered or unregistered. To deal with this, articles can be placed under semi-protection. Semi-protection is not a means to prevent vandalism from unregistered users but from users registered for less than four days and with fewer than 10 edits. Since there is no way to determine the length of time during which an unregistered person has been contributing (time-of-first-edit cannot be used because many different people may be sharing the same IP address), semi-protection consequently affects unregistered users in addition to newly-registered accounts. This doesn't mean that unregistered users are equated with novice users or that they are considered less trustworthy. As with the indirect creation of articles, anybody can suggest changes on the article-talkpage, for addition to the article in mainspace by an editor who can bypass semi-protection.
Edit from a blocked IP address or range: Registered users who persist in vandalism or disruptive editing can be blocked from editing by an administrator. Unregistered users who persist in vandalism or disruption can similarly be prevented from editing by the similar measure of blocking contributions from their IP address or range. If you see a block notice on an unregistered user's user page, remember that the person contributing today from that IP address may not be the same person who received the block. (Also, sometimes accidents happen, and the block was by mistake.) Similarly, innocent users (registered and unregistered) may be blocked from contributing because of a block placed on an IP address or range.
Directly upload images or rename pages: Like semi-protection, newly-registered users, and consequently unregistered users too, may not upload new files or rename articles directly. Unregistered users and users not yet confirmed may submit file upload requests here or request moves here. Outside official channels, they can also just ask someone they are already working with, or already familiar with, to perform the task. WP:TEAHOUSE and WP:HELPDESK are also useful places to get speedy help.
Directly use admin-tools, or become an admin-level contributor: This restriction applies in practice to 98% of registered users (as of 2013), as well as to 100% of unregistered users. Wikipedia withholds certain "buttons" from most users. These "buttons" are, for example, the ability to delete an article or block a user. In nearly all cases, it is the Wikipedia community that decides who may have access to these "buttons". The community decides whether a user can have these privileges based on evidence that they are trustworthy and exercise good judgement. Since many people may contribute from the same IP address, if these rights were given to an unregistered user there would be no way to guarantee that only that user would have access to the "buttons". For the same reason, unregistered users cannot be elected to a committee, such as the arbitration committee. As with the other categories, unregistered editors can always ask for assistance, from the nearest admin (or even the nearest ArbCom member). There are tens of thousands of active registered editors, but only a few hundred active admins (as of 2013 there were 80k of the former and 600 of the latter), so this restriction is not at all specific to unregistered editors.
Vote as distinct from the essential comment: On the few occasions when decisions (usually not content-related) on Wikipedia are decided by democracy (e.g. request for adminship, elections to the arbitration committee) unregistered users may not vote; they may participate in the discussions. Rather than being evidence of the untrustworthiness of unregistered users, this is in fact because of the untrustworthiness of registered users. If unregistered users were allowed to vote, disreputable registered users could log out of their accounts to vote twice (or, with use of an anonymizing proxy service, tens or hundreds of times). See also WP:SOCKPUPPET, which is a type of abuse where one human registers more than one username; detecting their underlying IP addresses often reveals such schemes.
As well as these restrictions, there are some specific advantages to becoming a registered user, such as watchlists. There are also some other, lesser used, limitations placed on newly-registered users that consequently affect unregistered users.