Getting this out of the way first, Awakening is an expansion, not a sequel. This means it's more about tweaks than major improvements, and you probably already know whether or not you will like the game. However, this article will give you an idea of what to expect and what has been tweaked and changed.
As you probably already know, Origins has a completely new setting and campaign. The campaign is a full new questline, playable with either an imported DA:O character or with a premade Awakening character. NPC reactions will be different depending on whether you import or not and what major decisions you made while playing the original. If you import you will also get a few exclusive cameos of past party members that may not show up on a premade character.
After importing you're left with the option of reworking your character completely, and redistrubiting all your stat points and specialties, or continuing with your current build and adding new ones as you go. The level cap has been increased and two new specializations have been added to each of the classes. This puts the total number of specializations at 6. Perhaps more important than this is that the current classes have been tweaked and balanced signifficantly.
Of course, there's new gear to go along with your higher level cap. 400-500 new items have been added for the expansion, including a much demanded stamina potion to keep your melee fighters active for longer. New enemies are prevalent, and of course with the different setting you can expect new dungeons and combat locations. Bioware wouldn't give me a total estimate for the amount of hours the game would take, but he said "a lot" depending on how much you enjoy sidequests, but not as long as the main adventure.
The actual gameplay hasn't seen many changes, but there have been some signifficant fixes for the console versions (which were actually made in-house at Bioware this time). Much of the slow-down is gone, but I still saw it on ocassion. The issue of the dialog being a little halting with too much delay between lines still exists as well, but overall the dialog is very good, and has plenty of the dark humor the original game was known for. The new characters shown in the demo were entertaining, and as with the original game you can expect to be given the chance to make major decisions which effect their development and role in the story.
There's not a lot more that I can say on the game, but I can tell you it's a lot of fun and a great expansion to an already amazing game. There's really no reason to avoid this if you enjoyed Dragon Age, and even the console versions are closer to par on this iteration. Look for a full review later this month on VGC.