Really quick, because I'm busy:
The new Dead Island really is more of the same. They've made a very few gameplay tweaks and added one new character, but that's it. I'd say the original is a better game solely because it's longer -- in this new one there's basically only two big areas (a jungle and a city), and once you do all the missions in those, it's over. I'm still getting some enjoyment out of it, but you really have to have loved the first game for this one to be worth it at all. Still, Dead Island is the melee equivalent of Borderlands, so it may still be worth checking out for some folks. My advice would simply be to pick up the first game at a bargain price.
As for Dragon's Dogma: Dark Arisen, I'm quite enjoying it. The combat isn't at a Dark Souls level, but it's still interesting and way better than the combat in your average action game. And the new dungeons have really added the difficulty that the original was lacking at high levels. Don't get me wrong, the original game is tough when you're low-level and still starting out, but they have a level cap of 200, and by the time you're lvl 70 or so there's just nothing that's really threatening anymore. I had stopped playing at lvl 115 because I was just demolishing enemies too easily. But the new area -- Bitterback Isle -- is a whole different animal. It's designed to be tough even for max-level characters, and it is (especially on a 2nd-playthrough, when enemies get even more badass). The pity is that this new area is fairly small and the rest of the world is still frustratingly easy, so that once you pass a certain level you're functionally stuck in a tiny portion of the world, unless you want to be just rolling over everything. Still a fun time despite this, though. I'm still on the prowl for new equipment, since there's a lot of stuff in the new area that's worlds better than anything that was in the vanilla game.