You never get horizontal lines across your screen? They mostly pop up in Safari when the web page is black.
Nope, the picture is absolutely perfect here (I just double checked with Safari on a black themed web site since I never use Safari normally).
Horizontal lines sounds like something I used to have on my projector downstairs (ground loop in the cable or something; it went away when they ran a new cable across my backyard to the box). I remember I bought a Pioneer Elite TV back in the late '90s that had black horizontal lines on DVD sources for some reason (more than a few people had that problem with that set, but it wasn't all of them). I never did find out the cause and they finally gave me a refund and I ended up buying a 57" Panasonic HDTV CRT projector for the same price six months later (it still works and has been running since ~1998).
I like mine, too. It's just the bugaboos in ML that cause me consternation.
That sounds like a hardware or connection problem (lines). Given your freezes, I'd suspect hardware all the more. If it were Mountain Lion's fault, everyone would have these problems. I haven't had a single crash yet on mine and it's been running almost 24/7 since November. It's normally up 24/7 too other than the occasional software update reboot or whatever. In fact, it's been up now for 7 days , 23 hours of this writing and that's only because I moved it over to a different desk last week. Otherwise, it would be more like 6 weeks without a reboot. As far as I'm concerned, this is the most rock solid Mac I've ever owned. I realize it's only been a very short period, I realize, but I've never had a Mac that didn't have some kind of crash early on. My PPC PowerMac was rock solid stable until Apple's software (usually iTunes) screwed it up with an update and then I'd get a stable version and it would run 24/7 for months at a time again. I think Apple breaks their own rules and causes more crashes than anyone, really (rare to see a 3rd party program bring the system down).
The Mini I have at home (not the one in 9400 model in my lab with SL) shipped with 10.8.2 on it, so it was a clean install when I received it. The only thing I did was use Migration assistant to bring it over. With my wife's
I used Migration Assistant too and I used it with my PowerMac running Leopard, at that. No issues.
ML is jerky on integrated graphics... So much sometimes I felt my workflow being interrupted... SL has much less jerkiness that this...
What integrated GPU are you referring to? It's smooth as ice here with the Intel 4000 at both 1920x1400 and 1650x1050 and that's driving two monitors at the same time.