By reading your signature, I can see you've got a core i7 processor and a SSD. Of course there are absolutely no downsides for you. lol. People with older Macs are having issues. I had no idea that AirDrop wasn't supported on my early 2008 MacBook Pro. That was the feature I was looking forward to the most. It just seems that I paid $30 for a ton of changes that I didn't like and had to turn off. I don't think I'll be getting a refund, so I hope 10.7.1 fixes some of the problems I've found.
I have a MBP with an i7 processor and there are absolutely downsides. There are so called tweaks and changes that were not documented, and some UI changes that you really just can't get a feel for until you upgrade. The fact that I can't modify my sidebar means that I can't see devices first for example. No one documented that you can't rearrange the order. A small gripe? Not really. Apple sold OS X in the beginning as the completely customizable user interface. Now spaces don't 'track' properly when you switch work spaces, the soviet style, grey everything interface makes it difficult to see things at a glance, and resume is a useless feature for someone like me who uses my computer for work, school, and my personal affairs. If you turn off aspects of this feature in the preferences then, then ctr+opt+cmd won't completely quit an app without loading your previous document by default. There is no work around. You have to save, close every open window, then quit the app for a clean use the next time. I almost never have to re-open an app for the same project twice in a consecutive period in a work day. I waited for Lion without pirating a preview copy of it, and like a little MacHead I paid the $29 for the upgrade, and now It takes nearly twice as long to do the same amount of work and I have to spend the time to back up everything and downgrade and I don't think a refund will come my way either. I have spent thousands of dollars of the past few years with apple and much more than that since my first OS 6 machine. The most disappointing OS X to date.