10 Nov 2004
24 Jan 2011
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My personal opinion is that Sitefinity 4 isn't ready for use as a community platform. Why would you want to recreate something like the Umbraco Forums when Umbraco has them already? Since Forums are not currently available in SF4 and based on current roadmap won't be available in Q1 either (mid april) you'd be waiting a while. If you have the time, resources and budget to customize SF to make it into something that supports a "community" then go ahead but that just doesn't make any business sense to me. I might be missing something about Umbraco and admittedly and not familiar with it, maybe there's a show stopping feature or two over there and that's why you're looking to see if it's a better option to customize Sitefinity.
As for the "CMS User" part of your question, there have been a number of posts on this subject as it's a pretty big issue for a lot of users with the new licensing model. In SF 3.x there was no limit to the number of concurrent "Backend Users" (note, this is the term Telerik uses for users that can log into /sitefinity/ backend and edit content) however in SF 4, if you look at the breakdown there are limits and they're rather restrictive. However, your "community" users would not have to be members of the "backend" user role. This also means that they wouldn't be able to use any of the backend tools to add/edit content so you'd have to create your own set of tools for the front end that took care of any and all editing. Again, I think this proves that for a community site Sitefinity 4 just isn't ready yet. Now if you could talk them into giving you a license for SF 3.7 you'd be in business, as it has Forums (simple but they worked) and unlimited backend users. Realistically I wouldn't do this though as I think they're in a rush to phase it out as it's not a money maker for them.
Hope that helps you out a bit. Anyone can chime in if I've got something wrong here but I think I've described things correctly.