I think the smartest move they made was letting Simon be the "bad boy/screw-up" character in a reversal from the first two. I loved his character in this, and I really admired Simon for going that dark in a comedy. And it was really sweet to boot, but not in any way that triggered my schmoop/fluff allergy, it was dark and bitchy and snarky---and somehow still sweet. And fucking hilarious.
The script was extraordinarily well balanced in letting the genre story (alien invasion) take time to reveal itself and keeping The Big Chill style bits go on longer than I would have thought possible.
Fun bits all around, great casting---yay Pierce Brosnan. Although I thought Martin Freeman was a bit too Marin Freeman-y.
Pegg's performance, including the pathos-inducing reveal were all brilliant, and that incredible affection that he's willing to show with Nick Frost, again without making me nauseated in the way many "bromances" do.
Great ending---with a nice genre switch and acceptable resolutions for all characters.
Highly recommended---I might offer to write this for Yuletide if it gets nommed.