Tried to activate SharePoint Server Enterprise Site Collection features on a new Site Collection and would recevie the normal SharePoint error (some text and an error ID). I grabbed the error ID and went looking in the ULS logs.

Here is what I found.

Feature Activation: Threw an exception, attempting to roll back. Feature ‘IPFSSiteFeatures’ (ID: ‘c88c4ff1-dbf5-4649-ad9f-c6c426ebcbf5’). Exception: Microsoft.Office.InfoPath.Server.Util.InfoPathLocalizedException: The InfoPath Forms Services support feature is not properly activated. Document library could not be found
at FormServerTemplates.

I went back to the Site Collection, created a FormServerTemplates form library (document library might have worked), then activated the feature again. And guess what, it worked!

Other tricks that worked for people:
http://social.technet.microsoft.com/Forums/en-US/sharepoint2010setup/thread/df474171-ee3b-4845-8e18-6a7119898bb9