The first was great, this is bigger, but not that much better!! A very good FPS, but could be more innovative.
Basic stuff is stayed the same, more or less the same weapons (some weapons added), the same enemies, but more intelligent and difficult to fight against, but all just better and bigger. And this is somewhat the weak pooint, it doesn't bring a lot of added value and misses the innovation and real improvement other sequels bring (ref AC II)
Landscape is changed and gives a lot more variation, you will have to be more inventive sometimes to get out, however sometimes your teammates will do a good part of the job also.
Storyline is not that worked out, further going on the fact that Nathan Hale has the Chimera virus in his genes, but without tremendous effects. This, while the other human who has got this is now the leader of the Chimera, called Daedelus (and very, very ugly)
The only thing I miss, compared to the first version, is the intelligence about the enemies. I can't find it anymore, which is a pity, because was always interesting to read and could help a lot in finding the best weapon to fight against this specific enemie. However, putting a lot of lead into them usually does the trick.
As a conclusion, just a great FPS, having all the elements a shooter needs to have and certainly able to keep you awake at night....