Regardless of disturbingly large number of rude and non-informative remarks, the question is actually quite good, and I wish I had had a compiled list of potential problems before I decided to make the switch on my iMac. So, here are my reasons for putting off the upgrade on my newer and shinier MacBook Pro:
1. Missing the Spaces/Expose (no, Mission Control is NOT the same thing, esp. when you have your iMac hooked up to TV that is not always showing your secondary desktop)
2. Digital Audio Output Problems -- (apparently, this "feature" creeped back into 10.6.8, too: https://discussions.apple.com/thread...art=0&tstart=0
3. Kernel error: nstat_lookup_entry_failed: 2
4. Kernel error: IOSurface: buffer allocation size is zero -- If you claim to have built a rock-solid system, can you please explain me how come some 3rd party player/plugin/whatever may crush the browser, let alone the whole system? I mean, whatever it is that (those idiots at) Adobe messed up... So, I have to agree with Graham Reeves: "I (REALLY) don't care if the problem is Apple or Adobe!"
5. Full Screen -- Quick time renders my other display useless when playing something in full screen (it's the same for all other applications, but, while full screen is new feature for them, QuickTime actually had completely useful full screen feature before Lion)
6. Performance issues (on my iMac 2008) -- boot takes longer, animations are not that smooth (they were flawless in SL), and it feels twitchy at times
This is not a complete list, just those things that bother me virtually every day and remind me that I should have been more cautious with .0 release. Now, if you feel like you're "missing out" the Launchpad, natural scrolling, prettier UI, improved mail, or some other features not present in SL (what are they, exactly?) and that they outweigh all the shortcomings, knock yourself out.