There are a number of UX issues with iOS 7 that I'm frankly quite surprised made it through testing or that anyone thought these were good ideas. Ignoring the theme itself (lower definition icons means less context, especially with hi res screens, that context would have been very usable it's the whole reason we do things like image previews for icons in modern OSes rather than generic jpg icons).
1) The "partial shift" no longer has a distinct visible mode on the keyboard. iOS has 4 modes for the shift button. 1: The button is off, everything is lowercase, 2: The button is on, the next letter or symbol is uppercase / shift symbol (? vs /), 3: The button is locked on, all letters are uppercase, but symbols are not shifted, 4: The button is partially on, the next letter is uppercase, but symbols will not be shifted. Mode 4 is the mode the button goes into at the beginning of a line or after a period. It was also previously distinguished by a blue highlight around the shift arrow rather than the arrow being filled in. Now there is no visual distinction between modes 4 and 2.
2) Minimalist button icons. For buttons that aren't text, the icons are very minimalist and without previous knowledge give little to no clue about what they do. For example the "share" button is now a simple box with an up arrow. The bookmarks icon in safari is a weird divided rectangle that if you squint just right you could argue looks like an open book.
3) The ".com" button is now hidden behind the "." key for web address entry making is non-discoverable except by accident.
4) Folders only display a 3x3 grid, even on iPads and do not remember your last position (nor does there appear to be an option for that).
5) When you first open the OS, it tells you that spotlight has moved and to now simply swipe down from any home screen. That's good, it's great that the search functionality is available anywhere. What it doesn't tell you is that you don't swipe down from the top (which gives you notification center. You instead swipe from another place on the screen.
6) The keyboard seems slower and less responsive. This may be just my iPad for some reason, but it appears that the keyboard sometimes hesitates on displaying and coming ready when displayed.
7) Videos have a "make full screen" button, but no longer have a "leave full screen" button that doesn't stop the video from playing. The "Done" button remains, but this stops the video. The only way to leave full screen without stopping the video is to pinch the screen.
None of these are show stoppers by any stretch of the imagination, but they are the sorts of "little things" that apple (and steve jobs in particular) are noted for fussing over. For making sure that those little experiences add up to be a better experience than the sum of their parts.