Here are some minor interface issues I have with Opera 10.6 on Windows 7. The first group was made during 10.5, but some of the issues still exist. I have crossed out anything that's been fixed by now:

1. As long as the chrome integration drag area default is so high, why not let the tabs slide all the way to the left, like they do if the menu bar is there? Also, the FF/key button shows a grayed out key when neither option is present, but it seems to me that a grayed out FF would be better, as that is the more common action.
2. There is not enough contrast between the active and inactive tabs when the stuff behind the window is white (it is not so visible here). Also, with all this space, why not write the page's title to the right of the Opera button?
3. Similar to comment 1: the [+] button should go closer to the trash can. There is too much wasted space in between.
4. This is the most minor, but press the search engine selector button. Then interact with the page. The button stays active, as though it were still held down. Only interacting with the browser controls will turn it back to normal. Additionally, the "Search with Google" text moves from the left to the right when the box has focus. This may be intentional, but seems like a strange decision, as the text clears right away when you type into the box, so bringing it to the right doesn't move it out of the way for anything. And if the right side is better, why not just have it there to begin with?
5. Four problems here. The first is that icon for the search is missing, and the generic search icon is in its place. This is old (from 10.10 builds). Second, There is no [x] to delete a search from the history (otherwise I would have replaced these chemistry searches with ones better for an example). Third, searches done in the search box don't show up in the history of the address bar and searches done in the address bar don't show up in the history of the search box. It makes sense to unify them, as some people (e.g. me) switch between the two randomly. Also, the icons for the two "previous searches" headings are different. The last is the drop down doesn't really look correct as a drop down, because of that space between the addresses and the address box and also the upper corners being rounded. This looks bad and annoys me. Also, the corner rounding is in general inconsistent. The address box should have the same rounding as the search box.
6. It is obvious what is wrong here and it has been pointed out before. However, I would also like to point out that the highlighting is a different color from the highlighting in the two drop downs.
7. The search icons for some searches sometimes fail to load from search.ini, and it is a complicated process requiring some luck to get them to show up. This problem dates back as long as I can remember, although I managed to get the correct icons on my 10.1 install. These were transfered by Link and did not get their icons with them, but I've had this problem in the past without Link involved.
8. This view box has two levels of transparency for some reason. The level of transparency is pretty inconsistent in this build. There are two here. The ctrl-tab dialog seems to have a little, but way too little to notice without close inspection. I don't think the two drop downs have any, but as long as the other two do, they should have the same amount for consistency. Two smaller issues around here is that the button's hover and active states only light up part of the darkened area and that that unnecessary dotted border sometimes surrounds the text.

The first is that when panel are open and tabs go to the left edge, the panel bar kind of blends into the leftmost tab when it is active. It would look better with a line separating it.
Next, the new tab preview make it impossible to check on downloads this way. Why not use the space to show progress bars for files. Similarly, the mail tab could show number of unread messages (I'm sorry if it already does that, my mail set up is in my 10.5 installation so I wouldn't know).
Also, I like the way that the top bar of tabs lines up well with the panel title bar. However, the downloads tool bar is a bit too big, ruining the effect for the downloads tab.
Similarly, the same thing happens on the source viewer tab. Also, the bar is not skinned properly, so its gradient is different from all other tabs and it doesn't change with the theme color.