First I'll say I won't bother going over all the rules changes. There are plenty of places on the net where to find out about that. This is post is basically going to say what I think of 6th.
I don't know where to start honestly. Overall, I think the new rules are excellent. Moving is basically the same... but as far as shooting goes; wound allocation, viable targets, focusing fire. All that stuff is great. The new assault rules are good too. I like the Look Out Sir Rules! and character challenges.
My only two things I had a problem with initially are pre-measuring being in (as judging distances was a learned skill) and random charge lengths. I am going to try and reserve judgment on both of those, as all the other rules are very cool.
New vehicle damage is awesome (Let me just say necron gauss weapons are KING once again) and makes sense, I like the passive nerf to krak missiles (and all AT weapons AP3 or worse, I'm looking at you psyrifle spam) and vehicle spammy lists in general thanks to the hullpoint system. Don't call me biased! - this is coming from a dude whose biggest army is mech angels.
I still have to look over the psychic powers more, but I like the different mastery levels having different shut-down capability, with even basic troops being able to shut-down on a 6. New missions are great, new cover system is great. Really, 6th edition, overall, is a major improvement. I am so glad that all the doomsday sayers were wrong. And if this is just Fantasy rules in space as some have been saying (I don't play WHFB nor know the rules), I could care less, because it's AWESOME.
Observations... 2+ armor saves are pretty dominant now with most specialist melee weapons being AP3. This also means regular terminators are going to be back in the groove, even though I've been running them throughout 5th. Calling it right now, Plasma guns and Lascannons are back for marines and Necrons just got a serious bump in army power. I'd expect to see them as top contenders at tournaments now, like Wolves and GK used to be.
More to come