My main problem with IE6 was that it was slow as molasses on my computer. Slow to start up, slow to render pages, but the worst was that it was slow to even *connect* to pages... the page would just hang there for a minute or more before it finally started to load. I finally got fed up.

Opera is fast in rendering, and it's intelligent with caching and connection-managing so it's fast at loading, too. Tabbed browsing is a total must-have... plus the ability to quickly load multiple links. And a built-in download manager. And a built-in pop-up blocker. And a built-in password manager. And one-click access to turning plug-ins, CSS, and images on and off. And a fully customizable toolbar. And a built-in mail client that's lots better than Outlook. And keyboard shortcuts for everything. And built-in customizable searches.
Not to mention that when IE crashes, it always took my OS with it. When Opera crashes, not only does it not take my OS with it, but it remembers what pages I was browsing so I don't lose my place. It's better at "remembering" form data when you leave and return to a page, too.
Not to mention that it has far more CSS and HTML support. There is a *ton* of CSS things, both flashy and useful, that you can do in Opera and Mozilla that IE can't. (Or it can, but you need to add on a crapload of javascript to do it.) And even the stuff it can do, it does differently, so more often than not you end up with a nice, clean-coded design that works great in Opera, Mozilla, and Safari... then you open up IE and you have to add a ton more coding to get it to work. I spend a lot of time swearing at my computer when that happens.

If you surf some of the "web designer guru" weblogs, you'll see
a lot of (justified) pissing and moaning about IE.
There's nothing wrong with being content with IE, I guess, but it seems to me like settling for runner-up.

Peace & Luv, Liz