Those two certainly don’t make your work easy. Whenever you float a div inside another div and also use negative margin, IE thinks it’s fun to not display it at all, unless you add position:relative; to the floated div. Only the newest version can cope without that. I suppose they still make some progress when developing it :).
When you have a link and a list in another list’s element like so:
<ul>
…
<li>
<a…>
<ul><li…>…<li…></u>
</li>
…
</ul>
… then you have to tell IE that it should display a-tag as block or else they end up on the same line. But if I remember it correct then IE6 couldn’t tackle my menu hovering at all, whereas IE7 does it well with some tweaking.
Oh and if any one of you has an idea why 10em border and 10em sidebar don’t have equal width in IE and Opera, do share your wisdom.