A few things I hate a bout IE8:

1) The per-tab process seems to slow down opening of new tabs.

2) Constant hangs, although being a beta, I can't really complain about that one.

3) I can no longer open new tabs right next to the current tab, as I could with IE7. And no, tab grouping is not the same, not even close.  And turning off tab grouping doesn't fix it either. In IE7 you had a choice about this particular behavior, now you are forced into one way only. I tend to browse like this:

Do a search.  Quickly scan down the results, and middle-click on any interesting link.  This way I find all relevant links first without losing my train of thought.  Then only do I look at the tabs that I opened. 

Once I have all my search results of interest in their own tabs, I look at each tab, one at a time.  Often I find additional links on the page that I'd like to follow.  So once again, I middle-click on all interesting links.  In IE7, the new tabs would open right next to the current tab, keeping everything grouped the way I want them.  In IE8, all new tabs are placed all the way to the right, completely losing context with where they came from.  And tab grouping doesn't solve the problem.  After a while, they just end up being opened somewhere to the right.

At the least, can we just get the "Open new tab next to current tab" functionality back as an option?  Please!  I am very close to giving up on IE and going with FF.  I have been resisting but it is getting harder and harder to stay with IE.