I've been using Ubuntu for a few years, but I'm looking to change because of 10.04 (hence I'm on the Mint forum). Here's some of my reasons:
1. My old Sony laptop that just started displaying the screen properly with the 9.04 release is now not working again. The boot-up and hence the virtual terminals (F1-6) display garbage.
2. Also on my laptop it hangs when I tell it to shutdown now. It closes all the open windows and then just sits there until I hard power it off. Again this never used top happen.
3. The login screen on my desktop displays at the resolution of my smaller monitor, so my larger (primary) one looks crap, and I can't work out how to make it use different resolution(s).
4. The proprietary nVidea drivers no longer seem to work correctly. They produce the same output as on my laptop (which has the old intel graphics). So I have to use the sub-standard nouveau, which means I have jerky DVD playback, and I believe is also the cause of point number 3.
5. They've messed up the 'application notification' area. I can no longer hide/restore Amarok, or adjust the volume, using the tray icon. To hide/restore I have to left click the icon, then select the option from the menu (I believe this is the same for any media player now). Also they have added a 'Me' menu that doesn't seem to be of any use at all, even if you do use IM, which I don't; it just gets in the way. The only way to get rid of it is to remove the package. Then there is the other new largely pointless menu which displays stuff I don't want, regardless of whether I want it. I don't use Empathy, but, I can't get rid of that or the IM menu items without removing the whole menu (again by removing the package).
6. I think this was actually in 9.10, but I used to be able to just plug in my Verizon USB727 mobile internet dongle and it would just work. Then they added a connection setup wizard and it stopped working properly. Everytime I wanted to use it I had to remove the old connection and go through the wizard again. This isn't too much of a problem because it's almost always plugged into my server and shared from there, but very annoying when we go on holiday.
7. A general issue from all releases: Removal of options from the GUI screens. For example the login screen option page. This now has only a handful of the options it used to have. You are stuck with the Ubuntu login screen and that's that, and if you want to change any of the other setting (remote logins?) you can't. Maybe they've moved this, but I can't find it. I assume this is to stop it being too scary for novice uses, but wouldn't it be better to just move it to an advanced tab? Or better still have novice and advanced user types.
The main thing stopping me moving to Mint is the likely hood that all these problems are still there because it's based on Ubuntu. I don't have enough monthly data allowance to download it just to try it out. I see that there is a possibility of Mint being switched to a Debian base. I can't help but think that would be a very good move, but only as long as they can keep the application packages up to date. Debian maybe solid as a rock, but it's also pretty stale.