I have heard that perfection is achieved when there is nothing left to take away. However, in a UI, you want the user to be able to find what they need in only a few clicks, without being annoyingly cluttered, so it would be better phrased as in UI design, perfection is achieved when there are no annoyances left to take away. Since no two users will find the exact same setup annoying in the same ways (much less two hundred million users), a degree of customization is required for every feature added.
Now, by that guideline, IE6 only has three major annoyances:
- Broken design means that most pages don't display correctly
- There aren't many useful features by default
- Web designers hate making sites for it (This is artificial, though, as by now they should have learned to simply ignore IE6, and let their sited break for it, naturally)
A fourth is added when a designer adds a popup telling you to upgrade, but these are seen as an annoyance of the website, not the browser. DO NOT USE, IT ONLY DRIVES USERS AWAY FROM YOUR SITE, NOT THE BROWSER. (hopefully they have gotten over it)
The thing that IE6 did right was to limit the features it added, so that there was very little UI clutter (and at the time, extra features weren't common in any other browsers, I would assume)
Now, IE7/8/9 have many more features all stuck onto the UI with very little you can do to remove the annoying parts (Tab groups: Either disabled, or coloured with an annoying palette. Being able to change the colour set would have been fine, but it's either on or off, making a useful feature into a mere annoyance), so, where IE6's annoyances are all caused by it's outdatedness, later IE browsers have annoyances from day 1.
Firefox has annoyances, but they can all be customized away, with a well-placed right click, or the UI customization menu option, or the right add-on. Now, if they make the "only tabs and one big button" UI the default, people will be annoyed until they get the old, cluttered, customized-to-perfection UI back, and if they remove the old UI entirely, then they will be as bad as IE7+, creating annoyances because *they* think they know better than the users.
As for IE6, when I used it it was because I accepted that, as an outdated browser, sites may break and if I didn't like that, I could always upgrade. I just decided that the UI was very nice and clean compared to IE7/8 at school (and the lack of tabs was a very useful defense against the tvtropes effect), since it had exactly what I needed, no more, no less, and nothing was obtrusive or annoying. Of course, after months of Firefox, I could never go back without finding the lack of a few features annoying.
So, a few tips for UI design:
If a user might not need it, don't clutter up their interface with it unnessecarily (or at least, in the case of a menu bar, have an easily accessible customization feature. Something that the new ribbon fad in microsoft products is lacking, aside from the ability to undock toolbars and move them to other edges of your screen), however, if a user might need it, keep it only a few clicks away. IE7+, from memory, would pop up a rather large icon when text was selected. I think it had the functions like "search for this" and stuff. Bad design, it's just clutter. Firefox, however, puts that stuff in the right-click menu, where you only see it if you are already going to do something fancy with the selection, so it doesn't obscure the page unnessecarily. IE6 just didn't do it at all, which, on my personal usability scale, fit somewhere in the middle, since copy-new window-google just took a bit more effort, and only appeared when specifically needed.
There, a demi-rant on what I think about UI design, and why IE7+ < IE6 < Firefox in my opinion, and how that might change if firefox discards the current UI.