Just installed Lion and right off the bat, I noticed bugs.
When moving the mission control and launchpad icons from the left to the right side of the dock, they both jumped back to their original spots. I had to move them a second time to get them to stick.
In launchpad, you click and hold on an icon to rearrange and delete, just like an iPad or iPhone. But, launchpad only allows you to delete apps downloaded from the appstore. Everything else, you need to navigate to the applications folder in finder to delete your app. Not really a bug, but an inconvenience.
Double-three-finger-tap on a word should bring up the definition of the word. This doesn't work in Firefox. I haven't tried Safari yet, maybe it works there, maybe not.
Scrolling is laggy every once in a while. I've never noticed ANY sort of lag in SL, and I shouldn't with my machine.
Fans take a bit longer to die down after intensive tasks, maybe Apple made that change as a safety measure, maybe Lion in its current state just isn't as efficient. Who knows?
The secondary click menu takes a bit longer to open compared to being instantaneous in SL. It causes me to attempt another secondary click which closes the menu

I know this also isn't a bug but it'll take some getting used to.
This is all from using Lion for no more than 10 minutes excluding the time I took to write this. I'll report with more stuff as I come across them.
Overall though, the first impression was a good one. I'm not regretting my upgrade, yet..