I've found a couple... namely with MSN checking my email inbox. Quite frequently it doesn't like my password that is remembered in MSN ... but then I enter the password again when prompted, and away we go.
Haven't installed this myself, but I would imagine there would be performance issues in this build like Sean reported. Perf tuning hasn't actually been done yet, and the IE PMs/Devs are encouraging people not to install the leaked build (for obvious reasons). Beta 2 should perform better, I'm sure.