This is a very unique category of web-based businesses: ones which keep their promises, but make promises which are unfavourable to their customers/members. The "good for me, bad for you" situation has existed long before the internet was even possible, but web-based operations have several distinct advantages for their owners, with anonymity being the primary one. With a website, it can be nearly impossible to reach the persons behind the website when things turn sour. Fly-by-night businesses are much easier to create and dismantle online. More permanent websites can thrive by slipping their unfavourable terms into a long Terms Of Use policy---which users rarely read in detail.

There is a definite need for warnings about these websites because, although they are legitimate, they can damage an artist's profession to varying degrees (either directly or indirectly). For example, a website which claims rights to artwork being posted on their site can cause copyright issues for an artist who intends to generate income through distribution licenses; a website asking for disproportionate fees for their services can create financial problems for artists who have limited budgets. The wording you want to look for is "worldwide, royalty-free and non-exclusive license(s)"...which means they are allowed to republish and redistribute your content, without limitations and for free.

A section in the forum has been created for artists to warn each other about websites which offer services with what they feel are unreasonable or unfair conditions (e.g. forfeits of copyright, unusually high fees). Artists are urged to review the stories and the information provided by the website to determine whether they want to do business with them.

Beware of these websites:

WetCanvas.com
Learn what Wetcanvas! does with your posted content and images
Although WetCanvas does not claim copyright to your content, their User Agreement Indicates:
"WetCanvas! does not claim ownership of Content you submit or make available for inclusion on the Service. However, with respect to Content you submit or make available for inclusion on publicly-accessible areas of the Service, you grant WetCanvas! the following world-wide, royalty free and non-exclusive license(s), as applicable:

• With respect to Content you submit or make available for inclusion on publicly-accessible areas of the Service, the perpetual, irrevocable license to use, distribute, reproduce, modify, adapt, publicly perform and publicly display such Content on, in connection with, or to advertise or promote, the Service, and/or related sites.
• With respect to Content you submit to the Reference Image Library, you agree that you took the photograph yourself and own all rights to the photograph you are uploading. You understand you will be held liable if you upload images that are copyrighted by someone else. By submitting work to the Reference Image Library, you grant WetCanvas! members the right to create and sell artwork, (including paintings, sculpture, etc.), using your photograph as a reference image in the creation of the artwork." (link)
  

BlogTv.ca
Globe & Mail review by Matthew Ingram
Matthew focuses on the fact that their Terms Of Use policy indicates that "All content and information on blogtv.ca including, without limitation, live broadcasts, on demand video, broadcast schedules, guides, promotional contests, images, illustrations, audio clips, and video clips (collectively, "Content") is owned or licensed by blogtv.ca and is protected by copyright and other intellectual property laws." (link)
I contacted blogTV.ca directly and requested clarification of their wording. I was referred to their Terms of Membership page, which indicates:
"You retain all of your ownership rights in your User Submission. By uploading your User Submission to the Website, you grant blogtv.ca a royalty-free, perpetual, non-exclusive, irrevocable, unrestricted, worldwide license to: (a) use, reproduce, store, adapt, translate, modify, make derivative works from, transmit, distribute, publicly perform or display such User Submission for any purpose via all forms of media now known or hereafter devised; and (b) to sublicense to third parties the unrestricted right to exercise any of the foregoing rights.
In addition to the grant of the above license, you hereby: (a) agree to waive all moral rights in your User Submission in favour of blogtv.ca..." (link)

Your moral rights include your attribution to the work and the right to the integrity of the work. For more information regarding moral rights, read the entry in Wikipedia.

YouTube
So that I don't seem to be picking on BlogTV.ca I thought I should point out that YouTube's terms of use are quite similar to BlogTV's:
"For clarity, you retain all of your ownership rights in your User Submissions. However, by submitting the User Submissions to YouTube, you hereby grant YouTube a worldwide, non-exclusive, royalty-free, sublicenseable and transferable license to use, reproduce, distribute, prepare derivative works of, display, and perform the User Submissions in connection with the YouTube Website and YouTube's (and its successor's) business, including without limitation for promoting and redistributing part or all of the YouTube Website (and derivative works thereof) in any media formats and through any media channels." (link)

Resources
American Art Law Organization list at the Artist Help Network
International Art Law Organization list at the Artist Help Network
Art Attorneys list at the Artist Help Network
Canadian Artists Representation Copyright Collective Inc