It was disappointing in the way that it introduced all these features that fans all wanted but unfortunately only scratched the very surface of them. Such as the Pred's vocal mimicry and the Alien's ability to blend into hive walls. Things like that were fantastic but they needed to be properly implemented.
The single player also repeated the problems with the original by being too linear and very uninspired. The Predator and Alien especially need more open levels to take advantage of their play styles. The Predator has to hunt and the Alien needs room to stalk and kill. It's very boring to play as the Alien, for instance, and only allowed to venture A-to-B through corridors and sewers for the majority of the game.
Regardless of all this, the mechanics were really solid. I loved the improvements to the Alien and Predator mobility. And it is an utterly beautiful looking game, even today. Just a shame it feels like only a taste of something better.
And as a side note, I had the most fun in this one on multiplayer.