A couple things I'm really missing after switching from Chrome: rounded tabs & the slight gradient background on the tool/bookmark bars. I just find it a lot more readable than the default square tabs and flat grey:
Also, I'm finding the spacing of bookmarks on the bookmarks bar is slightly wider than Chrome, so my imported bookmarks run off the edge of the bar.
On Firefox, I was able to use the excellent "Stylish" extension to edit the guts of the interface and make FF look almost identical to Chrome. Although Stylish exists for Opera, it's for v12 & doesn't seem to work with v22.