I have gotten quite a lot of feedback from folks asking why we built a SharePoint Community Site on DotnetNuke and not on SharePoint. I have a lot of experience with both SharePoint and DotNetNuke and for the most part, my experience tells me that SharePoint is GREAT for collaboration and is wonderful to use as long as you have 2 things. Fast Connectivity and User Accounts.

In the current version of SharePoint, each user for SharePoint must also be a user on the SharePoint Server. This leads to licensing issues when the site is hosting a large communty. DotNetNuke doesn't have any of those limitations from a Licensing standpoint.

DotNetNuke is a good alternative to SharePoint for external (Extranet) sites but shows its weakness in the Document Collaboration / Microsoft Office Integration area. This is the area where Sharepoint shines.

We chose DNN for now and will in the future move to Sharepoint for hosting our site if we deem it ready for external collaboration over potentially slow connections. Can't wait until v3 comes out :-)