The whole Distributed thing of Git is really really great. it gives a few features Shelvesets don't have (in the current product) such as local rollback and commit options (such as Eclipse's localhistory feature). You could alleviate this using developer branches, but lets be honest, many developers don't like branching and merging one bit. I've been asked to turn on the old style "exclusive checkout" feature in TFS a few times too often (and denied it each and every time).
I think many large enterprises are quite scared to allow a dev to just bring the whole history into a local workspace and take it with them (to a new employer for example)... Stealing a snapshot is bad, but taking away a whole history is even more troublesome. (Not that you couldn't get a full history from TFS of you wanted it)...
It's mentioned that it's a great way to backup, which is great for open source again where the original maintainer might stop to care and removes his version, but for a enterprise plan this again falls short for many enterprises as there is no clear assignment of responsibility to keep backups. And it would be hard to figure out which version to use if the main 'project' vanishes somehow. Which would tend to appoint one repository as leading/central.
What I like most about Git is the Push/Pull option, where you can easily contribute code to a project without the need to have commit rights. I guess you could use very limited users and shelvesets in TFS to mimic this, but it isn't as powerful as the Git option. Branching across team projects might work as well, but from an administrative perspective it's not really feasible for many organisations as adding team projects adds a lot of administartive overhead.
I'd also like to add to the things mentioned in the non source control area. Features such as Work Item Tracking, Reporting and Build Automation (including lab management) greatly benefit from a central leading repository. These become a lot harder when you use a pure distributed model, unless you make one of the nodes leading (and thus go back to a less distributed model).
With TFS Basic coming with TFS 11, it might not be far off to expect a distributed TFS which allows you to sync your local TFS basic to a central TFS in the TFS 12+ era. I'll put my vote for that down in the uservoice!
