1) The option of a dark background in grid view has been removed; the white background is horrible;
2) Sometimes the expanded view works well (the algorithm is clearly quite clever, zooming in to remove borders, etc.), other times it looks atrocious; it's around half and half; I've now spent hours debating whether to turn it off or keep it on;
3) Track lists in columns, everywhere, often inexplicably (e.g. on an artist page with no other albums); I know there's a bunch of horizontal white space to fill, but this is an ugly way to fill it;
4) With the new grid view and 'expanded view' there is no longer a quick way to view useful metadata about tracks (e.g. last played, number of plays, etc.);
5) Previously track lists formed 'rows' that had alternating background colors (I forget the name for this) that made it easier to follow; this is no longer the case, and for some reason looking at track lists in iTunes 11 makes me want to break things;
6) The new status bar no longer allows one to toggle the unit of measurement for the amount of music in the library; previously you could toggle hours, minutes, etc. now all it shows for me is days;
7) There is no longer an easy way to copy album artwork out of iTunes; previously you could have a preview up in the sidebar, right click -> copy; now one has to select an individual track (inexplicably, doesn't work for entire albums), right click -> get info -> artwork -> copy;
8) The album artwork thumbnail at the top of the window (in the little dialog showing the track, etc.) looks ugly and can't be removed;
9) The sidebar in the old version used to match the sidebars in other OS X apps (Finder, Mail, etc.); now the font has changed to Helvetica (bold, for that matter) it doesn't match anything;
10) The coloured sidebar icons that got ditched with OS X Snow Leopard and iTunes 10 are back; I really hope this isn't a sign of things to come elsewhere in the OS;
11) The option to change the size of album artwork in grid view has been removed; it's now one size, take it or leave it. This is excruciatingly irritating;
12) Still no FLAC support; this is beyond a joke now;