1. The only major "upgrade" in mountain lion is the notes and reminders apps, which sync with an iphone. I rely on exchange calendaring, notes, and reminders on my iphone, but since I'm quitting my job, I will no longer have exchange. So iCloud does a pretty good job of replacing exchange for me.
2. The app store in mountain lion really has little useful software for me. It might be good for games, but not much else. The nice feature is that all the apple apps as well as a few others such as drivers update through the apple store. A lot of these don't require restarting your computer, so it's a little more convenient then snow leopard.
3. mountain lion has support for windows 8, although you can just download the drivers from apple's website. They used to force you to use bootcamp to download windows drivers, but they don't do that anymore.
4. Finally, if your computer is slow, and you're thinking about reinstalling snow leopard, it's would be the best time to install mountain lion and would give you something fun to play with for a day or two.
Now for the bad:
1. Windows XP and vista support dropped by bootcamp.
2. You can no longer access display preferences from the menu bar. You have to go through system preferences or press command + F1, which is embarrassing during a presentation.
3. Power PC support dropped, although universal apps should continue to work.
4. Gestures on the trackpad totally screwed up, although if you have an imac, i guess you probably don't have a trackpad.
5. You won't get your iphone photos synced through photo stream unless you buy the latest version of iPhoto through the app store. The snow leopard version you have won't work.
6. Notification center doesn't do much useful. I thought it might be like an iphone or ipad where you could get updates like ESPN and news updates without having the app open. It's nothing like that, and there are few apps that take advantage of notification center. Maybe twitter and facebook do, but I havn't installed them yet.
7. Spotlight indexes a lot for some reason.
8. Mountain lion with a ssd is the same speed as snow leopard with a hard disk. That probably indicates mountain lion with a hard disk will be slow.
9. Finder hides the user library by default. You can install a hack or temporarily press the option key to see the user library, but I think that's annoying for power users.
10. The finder itself has a lot of useless bloat on the sidebar and it's all in grey instead of color. That makes it slower to use then the snow leopard reader.
11. Preview no longer has the hand (move) tool. So it will highlight text instead of scrolling with a mouse. This is not a big deal if you have a laptop as the trackpad scrolls for you, but if you have a mouse this is a major annoyance. The scroll bars are hidden by default, until you scroll with a mouse wheel or trackpad, which means to scroll by dragging, you need to first scroll with mouse wheel, then move the mouse to the scroll bar, activate it by hovering like an idiot, and then scroll with scroll bars.
12. Opera (from the app store) crashes a lot more then it did in snow leopard.
As you can see, mountain lion removes a lot of useful features of snow leopard and dumbs it down, and adds few useful features. Mountain lion can be really unstable if you upgrade or clean install and import old preferences. But mountain lion seems very stable if you clean install the OS and all apps.