There's virtually no difference between 2.0 and 1.5 because everything 2.0 brings to the table, 1.5 supported with extensions. Some of you mentioned "session restore." I believe that was one of the features of the Tab Mix Plus extension. The X on the individual tabs were also part of Tab Mix Plus.
Some miscellaneous extensions like Disable Download Targets aren't compatible with 2.0, but in this specific case it seems like 2.0 itself has that feature built in, minus the customizeability.
I'm not bashing on 2.0, I'm just saying it's not as big of a change as I had expected. I just went from IE6 to IE7 and that was one hell of an overhaul.