Either way, mapping the arrow keys to <nop> seems wasteful. In normal mode I map up/down to :cprevious/:cnext and left/right to :colder/:cnewer, which makes error list navigation effortless. Some other good targets are the buffer, tag and location lists (with a little work you could make them context-aware too). I don't remap the arrow keys in insert mode though, it'd be annoying to have to exit insert mode only to move the cursor a few characters.