Probably the achievement for completing Gears of War on Insane. (Still working on GoW2) It took a lot of work and I was about ready to give up on a couple different sections, but a friend and I finally pulled it off.
Definitely Final Fantasy games. I've played them on every available system since the NES, although I won't say I've played every single game. So, take the whole main FF catalog minus a couple games, add in the Tactics series and a couple Crystal Chronicles titles, and you've got a massive investment of time/money.
Absolutely not. First, they've been out of the market too long. Second, with OnLive on the horizon that's going to bring us to a total of 4 major consoles (not counting handhelds). I'd rather not have a 5th to deal with.
I've never let achievements stop me from playing a game. If anything, ridiculously difficult achievements only stop me from working for that particular achievement. Looking at the example you've given, I'd suggest that maybe achievements are just *slightly* too important to you. If you've chosen not to play an excellent game like Halo Wars just because you know you'll end up at 950/1000, then you're doing yourself a great disservice.
I'll laugh as easily as the next guy if somebody griefs me once or twice. Within limits, it's funny and it's a slight deviation from the norm of the game. It's when people start griefing me or someone else over and over and flat out ruining the experience that I start to take issues with it.
It works both ways. Wii MotionPlus is going to allow sports games (all games, for that matter) to be more responsive, but some of the responsibility falls on developers to make sure they integrate the technology properly.
Honestly? I don't think they're all that great. I camped overnight to get a Wii on launch day, and I kept it until just recently. Over and over, the games available for the system proved to me that while motion control is interesting, it's not enough to base a system around.