The voice acting is quite good, despite what they're saying being a very thinly stretched plot. The amount of audio exposition that happens after the level is beaten, messes with the pacing. I understand why they did it -- it gives you something to listen to, while checking your scores against other people, but it makes the level that you finally beat with a gold medal require you to wait a few more minutes before moving on. I only played the game on the Hard difficulty, so by the time I finished the level, I was kinda done with it. There is also a lot of text that is given to you at the start of each level, which will not make sense for over half the game.
They added three new features to the game, and removed flying units -- which helped focus the game to a ground based defense model. The first is boost towers, which allow you to cheaply route the enemies without having to build a lot of unnecessary gun towers, and they also allow buying a boost to the tower that you build on top of it (more damage, more score for enemies killed around it, or stealth detection). It was a nice touch.
The second thing they added was upgrades that you have a chance of getting at the end of beating a level. The upgrades affect a specific tower, and you can choose which upgrade to put on each of your towers before starting a level. My personal favorite upgrade was the tachyon ones, which slow down enemies when you hit them. Second to that was the one that changes the targeting to hit the strongest unit, which was great for cannons or missile towers.
The last thing they added was the ability to purchase extensions to the map, so you have more places to build, or more paths to route the aliens. It's not always clear how it's modifying the map, but after a few quick purchases and rewinds, it all makes sense.
I'd highly recommend picking it up. It was an improvement to the first.
Also, if you're confused on the new scoring system you should check this discussion. Defense Grid 1 used a rating system of remaining currency + X per core you still have + sell value = total score. Defense Grid 2 is similar, but not as transparent. It makes sense when you think about it. You score better if you can build a more efficient defense.