The last word on Mountain Lion: SUCKS...!
I thought it would suck, but other than an issue with Samba (that also exists in Lion due to Apple dumping it and rolling their own that works fine with Windows, but doesn't work with all 3rd party stuff like XBMC, etc., but was remedied pretty quickly by using "SMBUp" to switch to Samba3), I have to say overall, I PREFER Mountain Lion to Snow Leopard (one other thing that sucks is the loss of Rosetta, but I've gotten over it since I replaced both my two Apps I used to use and now have a Wine wrapper for Diablo II (VMWare Fusion lets me use Win98, WinXP and even Linux with 3D acceleration in OSX anyway and it really plays games that are a few years old just fine without Boot Camp).
I thought I'd hate Mission Control, but I actually like having Expose and Spaces combined (one hot spot saves me a load of trouble and App Expose is available with "Show Windows" from any Dock icon, so there's no issue for me. I don't like how Full Screen works with two monitors, but two monitor support has ALWAYS sucked in OSX and it's no better in Snow Leopard. At least you can get the menu bar on the other monitor when its in Full screen mode for that app (slight improvement). The real problem is that the other screen is empty while it's in full screen and that needs fixed (but same thing in Snow Leopard except the screen is black instead of textured).
Launchpad is mostly useless (could be made to be useful at some point, I think), but thankfully it's not forced on you. You can just remove it from the Dock and safely ignore it if you don't like it (in other words, if it doesn't hurt anything, who cares?)
I really like Notifications. Not only do email summaries and other program messages pop up while active, they're stored on a menu-item drop-down to look at if they come up while away from the computer.
OpenGL has at least been updated to 3.2 (a real improvement, although they should be at 4.x by now, really)
Frankly, I'm hard pressed to think of significant other differences that come to mind offhand. I'm sure there are some, but obviously they're not an issue or I'd notice them. Some things like iTunes 11 (hate or not) have nothing to do with Mountain Lion (I'm still using iTunes 10.6.3 here). Some default settings changes can be easily adjusted to behave like before (most of mine got changed automatically when I moved my old PowerPC stuff over; yeah it moved a PPC Mac just fine other than some unusable PPC only Apps that I just cleaned off. Even Photoshop CS3 worked fine after it was moved. All my GUI settings, configurations, documents, emails ,etc. all moved over just fine and of course my quad-core i7 is like freaking lightning compared to the old PPC machine (even though it was upgraded to 1.8GHz and had a Radeon 9800 Pro in it and a Sata II interface and very fast hard drives). The Intel 4000 my Mini came with isn't great for modern gaming, but older games and all HD stuff, etc. and two monitors work great here (the two monitor setup could use some improvements as I mentioned earlier, though).
My new Brother printer (MFC-J835DW) works fine with it as does my older Brother HL-5250DN Laser. The Memorex Blu-Ray/DVD/CD 12x/16x/40x USB3 writer worked immediately without issue and my Western Digital USB3 hard drives I had been using in Leopard with USB2 worked immediately at the higher USB3 speeds. My Logitech 9000 webcam still has crappy frame rates as always at 2MP resolution (with no OSX option to use a lower/faster frame rate), but at least it works for FaceTime and Skype calls, etc. In other words, all my old hardware accessories worked fine in Mountain Lion.
Really, I have few complaints except the dual-monitor operations need work and the Finder still sucks big time, but for that I have found a great remedy. It's called "XtraFinder" and it's free (unlike some of the VERY pricey commercial ones out there). It has an awesome tabs and dual-pane modes and even a way to just automatically create two separate side-by-side or top/bottom sets of finder windows spaced next to each other with a menu click. It works great with Mountain Lion (no issues other than the option to use "color icons", which seems to cause a big lag/slow-down so I leave it off for now, which is too bad since color icons in the Finder look SO much nicer than B&W. They've made iTunes color again; they should make Finder color again too.
Oh yeah, I forgot about iCloud. You don't have to use it or you can only use parts of it. Frankly, I find it pretty slick to sync bookmarks, notes, etc. automatically with my iPod Touch 4G and even my account on my mother's Macbook Pro at her house (and my own Macbook Pro if I upgrade it from Snow Leopard to Mountain Lion soon).