You start off as a human rogue/warrior/mage and you cannot choose another race unfortunately. Now if you played Origins, you will notice a bunch of recurring characters and you will also understand things a little bit more, but it is not necessary to play the prequel to enjoy this game in my opinion. You will always get dialogue choices, which will usually add friendliness or rivalry between you and your comrades, or sometimes will change the outcome of events. There are often some funny options (although they can lead to unexpected results) and you are free to experiment and be a real asshole if you want to - you'll still be the "champion" at the end :]
WARNING: the game might confuse the shit out of you at the beginning, since it's hard to understand the relationship between the separate events and the role of each character in the storyline. Eventually, though, the puzzle pieces come together and you should understand what's happening, so HAVE PATIENCE.
One thing that slightly annoyed me was that most of the themes are the same as in Origins: the Darkspawn threat, Paladins VS Mages, political unrest, some more Paladins VS Mages stuff, betrayal, etc. At some point you just feel like saying "Oh great, another quest about paladins and their eternal enemies mages, sigh." Oh and if you actually read every single history related thing you come across (also called codex entry) then you've officially reached the highest level of geekness: those are often really long but if you're really into the whole mythology of these series, by all means go ahead. As for me, I wanted to play a game and not read a book.
This was probably the most successful part. First of all, many races and faces were completely redesigned (the Qunari, a very manly and stoic race, look much cooler now) and animation in general is smoother. There is a good amount of cutscenes and the battles have a much higher variety of moves and effects in general. I loved how blood splashes everywhere during battles, especially for rogue moves. And after the battle, if there is a dialogue, you see the camera zoomed on your characters' faces covered with blood and talking to each other with a completely normal expression as if nothing was wrong (made me lol several times).
Battles are very fun. The geography and the positioning of allies and enemies will often yield interesting situations. You'll want to pause as often as you can and figure out where to move, which abilities to use
It brings us a bit less than we'd expect from a sequel, but is nonetheless a great epic and if you loved Dragon Age: Origins, you'll love Dragon Age II.