I noticed this little issue after doing my last SharePoint 2010 upgrade. The client was a fan of the Tag Cloud Web Part on his My Site and wanted to bring that functionality onto one of his team sites. I figured this would be no problem, so we went to the Team Site, clicked Add New Web Part, looked under Social Collaboration only to find the Tag Cloud missing from the list. In fact, Tag Cloud was not the only thing missing. Contact Details, Note Board, and Organization Browser were also missing. I checked the web part gallery and there was no sign of the Tag Cloud present. I verified that the SharePoint Standard and Enterprise features are activated, but still no sign of it.
At this point, I decide to do some digging. I wanted to find out which feature installed this web part. After looking at a working SharePoint 2010 server, I determined that the web part comes from the file TagCloud.dwp (a version 2 web part schema on a brand new web part? Really?). I head to the 14 hive and search the features folder and determine that, this web part is deployed by the PortalLayouts and MySiteLayouts hidden features. Then it all makes since (well sort of). Since I upgraded this site from MOSS 2007, the new version of PortalLayouts has not activated on my site collection.
At this point, I had a decision to make. I could force the PortalLayouts feature to activate again or I could leave well enough alone and just manually upload the .dwp file into the Web Parts gallery. I went with the latter so I grabbed TagCloud.dwp from 14\TEMPLATE\FEATURES\PortalLayouts\DWP and uploaded it to the web part gallery and the web part worked just fine. I wasn’t sure what the other unintended results might be on the site collection had I force activated the PortalLayouts feature again so this seemed like the right decision. If you are interested in the other web parts, you can upload contactwp.dwp, profilebrowser.dwp, and socialcomment.dwp to your gallery as well. There may be other web parts missing too, but I think this is it.