Properties, coming to TFS soon!
One of the things I miss when using TFS instead of Subversion is svn's metadata ("properties") that you can attach to arbitrary items ("svn propset" for the fellow command-liners out there). It's something I only used around 10% of the time, but when you need it, it's very hard to live without it.
As Brian mentions in this blog post, we have a spec for properties in our upcoming version of TFS. As you can see from the list he mentions, there's a lot of scenarios this enables (his list is very much the tip of the iceberg) that are very difficult without it. I'm glad to see we'll be getting this and hope lots of people give feedback, especially any Subversion users that miss that feature and want to make sure we handle their use case :)
While Subversion uses these properties internally, I think it's worth pointing out at least one scenario that a common svn client (TortoiseSVN) supports: linking with bugs in Trac (or another bug-tracking system).
That particular scenario is handled natively in TFS with our links between changesets and work items, of course, but hopefully it makes it clear what kind of power properties can deliver to tons of scenarios, especially those around integration.