The only serious problem I had–have–is that I can no longer get my WPA-encrypted wireless connection to work. Of course, that’s a bit of a showstopper–I am paranoid enough not to turn off encryption (and so should you be). I have just filed my first open-source bug report. Ah well, that’s betas.
The wireless problem may well have prevented me from running into any other major issues. I have hit a couple of minor ones, though. Most seriously, terminal fonts looked dreadful; that turns out to be known and easily fixable.
Other than some graphic coolness the new feature in Hardy I expect to find most immediately useful is wubi, which looks like it ought to make it possible to install, or at least test, ubuntu on a windows machine without doing scary things (well, disk partitioning tends to scare me) to it. I have just such a machine that I’d like to ubuntify, and I’d really like to make sure its wireless connection works before I do anything drastic.