Before ditching an OS, I would certainly want to make sure that any problems I was having were actually directly caused by the new OS, rather than merely "something happening on my computer right now."
I haven't noticed any slowness in startup -- perhaps shutdown is slower than previously, but if I've pressed the shutdown button, I'm already moving out of the room.
You should never argue over taste, of course, and some people just "like" Snow Leopard. Personally, my experience is that most of the OS has the same features and functions, although there are some very nice additions and fixes, as well as some interesting new features and under-the-bonnet improvements.
Things like the ability to merge folders when copying in the Finder, for instance. AirDrop is also very useful for transferring files between Macs without having to have Sharing on all the time.
I like Resume -- I turn my Mac off every night, and when I start it up in the morning, it's exactly as it was. Apple is moving towards an OS where you don't need to know or care whether an app is actually loaded into memory before you start using it -- this is the only real "iOS-ification" of OS X.
I also like the new Documents method. I've been using Macs for twenty years, and yes you do have to change your habits a bit, but once you get used to it, it's a real boon. Finally free from the tyranny of having to hit save every time I do something! Versions is also very useful.
To say nothing of iCloud, which makes syncing Calendar, contacts, reminders, Notes, Photos and even documents across two Macs and an iPhone effortless. And you get another email address.
Even LaunchPad, which I thought initially was just a gimmick from iOS, is pretty handy for launching apps when you've got two large monitors and the Dock is miles away. Just make the gesture, and click on the App.
This is just a fraction, and much of this was introduced in Lion, whose benefits you also get if you're jumping from SL. (And with ML, you get the refinements on Lion's features.)
If anyone really wants to know about ML, then John Syracuse's detailed review on ArsTechnica is essential reading:
http://arstechnica.com/apple/2012/07/os-x-10-8/