I recently upgraded a computer to Ubuntu 12.04 LTS: precise pangolin. The update process was smooth and simple, but the finished install had a number of failings. I proceed to gripe:
The install attempted to migrate my previous settings. It failed. It reset a number of my customized keyboard commands, which was annoying to notice but easy to fix. I'm still not sure I caught them all -- I'll just find out when I try to use some command and it fails. Also, the update managed to copy over my user account and files, but also created a new guest account with no password. What a terrible choice! For security, and privacy! This makes me suspect that I need to closely examine every update for giant security holes so large that the developers look right through and past them. This, too, was easy to fix.
The difficult-to-fix things are all features of the new Unity interface. The Ubuntu online-social-networking suite of programs autostarts on boot up, and has a big envelope-shaped indicator permanently on the top bar. The main menu has been replaced with something called "dash" which is a search -- no more browsing installed applications! -- and prominently includes programs found online which you haven't installed yet. The "launcher" -- formerly the task bar -- features non-differentiable icons of both shortcuts and currently-open programs, and also includes some icons which cannot be removed without recompiling or switching to an experimental build. The alt-tab menu inexplicably always contains the option "desktop."
In an attempt to be more user-friendly, there's no easy way to edit these settings. I've spent two evenings now trawling message boards and developer forums. I installed a configuration tool that came with a ridiculous number of warnings about borking the entire system. I managed to remove the social-online-networking stuff by forcibly uninstalling the entire suite of programs. I fixed the alt-tab menu by rolling back the entire alt-tab--bound keyboard system to a previous, and now tenuously supported, package. I can't find a trustworthy way of fixing the remaining things, so I've established workarounds whereby these features are auto-hidden and not used by default.
I've fiddled with it to be Lila-usable, but (1) it took awhile, and (2) it provoked me. Why would a pangolin do such a thing? All I want is to alt-tab reasonably between open windows!
Ultimately, I don't like Unity because it seems anti-unix. It discourages learning about your own computer by making settings hidden and unchangeable. The "search-only" menu is especially bad, repressing exploration.
This post's theme word is noisome, "offensive (esp. to smell), harmful, noxious." This experience has soured me on the pangolin; although I've never seen one, I consider it among the most noisome creatures imaginable.
[Update 9/12/2012: Found more broken keyboard shortcuts. Also, hibernate is disabled to the point of being not even an option by default? ... because some users have problems with it? Boo. Fixed and fixed.]