SVN essentially lets you version filesystem trees. And it will let you version disparate filesystem trees in the same repository. We've got alot of deployment bits we have handled through SVN with different parts of trees on different servers, etc. I'm starting to get to a point with DCVS systems where I can see some correct ways to model this, but there are still some stunts that you can only do with SVN.
Another angle is security -- some of these devices are pretty close to the edge. With DCVSes your entire repo history is there for the taking. And, if you are on a cloud provider where you are paying for space and backup, them DCVS repos get pretty hefty.
Final angle is some file size issues. We've got a few old projects with some large binaries in SVN. Since SVN is onsite, this is managable. We have tried to import these things into HG a few times and it has always crashed and burned due to some of the large files timing out the network. I probably could get it to work over ssh, but that is not something we want to take on.
TortoiseSVN is polished to the point where I now rarely touch the command line unless I'm using Linux at home. TortoiseHg is good, but it still doesn't feel as usable as TortoiseSVN, especially when branching and committing.
Funny, I think the branching / merging / committing parts of TortiseHG shine compared to TortiseSVN. Parts that I think need help are some code merging dialogs and the shelving stuff. The other trick is updating -- it isn't as in your face about it so I do it less often than I would TortiseSVN. But they are making major gains in every version.
I've always liked GitHub as a company, and their app looks fantastic. As a C# programmer I was very interested in reading how they built the tool, but I am still yet to use it as I'm not a big fan of having my code out in the open (which is why I use BitBucket for personal stuff).
Also, and I know this sounds a bit crazy, but when a developer tool looks THAT nice I get concerned about whether it'll actually work.
I think it was a land grab towards those impressed by pretty. And also aimed at how bad a story that git is on windows. Too bad those guys are so totally sold on git, really no reason they could not offer HG as well -- logical model is similar enough that all the tools should be able to work with it.