I like Merrill, though a great many people will find her to be the most annoying character there. Personally, that honor goes to Anders.
The storytelling in the game is different, though that's not exactly a bad thing. If you go into this game looking for a directly sequel to everything that happened in Origins, you're not going to like the game. If you go in understanding that this is a game set in the same universe, with the occasional mention of previous events, but not linked directly, then you should be fine. Easier way to say it: This game is not 'Mass Effect 2 and 3'. It doesn't continue like those two did.
The combat, at least for me, felt more enjoyable. More weight behind the hits, so to say. However, respawning enemies out of nowhere during a fight does mess up tactics.
But the thing that I like most is the lore. There are some serious plot revelations and hints running through Dragon Age 2, but more often than not they take a back seat to what's happening to Hawke. Example: a discovery is made that completely changes everything that's known about a certain race. However, this is quickly cast aside because of the immediate problem of getting out of the Deep Roads (and saving someone, if you brought certain people with you). There's also a series of hidden messages that you can find throughout the game, and the final one reveals that something really, really bad has escaped the Fade. Again, this is never explained because the city is burning around you when you find it, so Hawke is more concerned about other things.
Dragon Age 2 sets up a lot of possible story points that, if the writers do things well, will make a mind-blowing story. I've already come up with my theory on stuff, so I'm waiting to see if I'm right.
Should you choose to get this game--and I say you should, especially if you can find it for cheap--I suggest picking up the two DLCs Legacy and Mark of the Assassin. Legacy if you can only get one, because Legacy takes those storytelling things I told you about (hinting at something greater going on) and dials it up to 11. There are some serious questions raised in that DLC. And I enjoyed Mark of the Assassin myself. It was a fun little trip, and it gets out out of Kirkwall.