It's kind of "more of the same thing". It plays almost identically to the first game.
It's always cool when they give you a huge open world sandbox game where you can explore to your hears content, and then fall into the missions as you wish. So that's already a bonus for me. Moving around the city is awesome and most of the times really seamless. There are those times when you just can't seem to get him to stop doing backflips though.
They's added a lot more you can do like Blacknet missions, memories, random upgrades, and all kinds of collectibles that will keep you busy for a while. Actually, the things that really kept me busy were the little challenges. I was OCD to get Gold in all of them, and I did. So yay for me.
The one thing you immediately notice though is the upgrade in the graphics VS the old game. It's much more detailed and you can see much farther than before. There is quite a few new upgrades to make you character even more unbeatable.
At the end it's pretty much just Prototype 1 with a new overcoat. It doesn't really impress, but it still is just plain fun. If it ain't broke, don't fix it.