Though short of revolutionary, Ep. 2 combines all the best aspects of its predecessors and perfects them enormously.
If you are a longtime fan of the Half Life series, you'll find nothing new in Episode 2. You will find cutting edge graphics, physics, and sound, excellent pacing, and some jaw dropping scripted events. The difference between Episode 2 and its predecessors is that Valve has polished all of these traits of the Half Life series to near perfection. The pacing keeps things constantly exciting (with the exception of the games second level, which gets a bit repetitive) and varied as well. It is evident that Valve put a lot of work into timing their action sequences and puzzles just right. Furthermore, these are both executed superbly and instead of the typical "trip this switch, kill this boss" gameplay, you'll find yourself constantly being put up to different tasks.
Despite the Source Engine's age, the visuals in Episode 2 are nothing short of stunning and are still, to some extent, cutting edge. Some new games may have slightly better technology, but what's important here is that nobody implements its technology like Valve. Some of the outdoor scenes are just gorgeous, and make me wish Oblivion had been created with source. Everything from the facial animations, to the new particle physics that cause gobs of blood and ooze to spew from headcrabs is implemented perfectly. The physics too are even better than they were in Episode 1, and several scenes throughout the game are stunning merely for the fascinating physical events that unfold. Some of the panoramic vistas will even catch you by surprise and you may find yourself stopping to take a look. On of the major criticisms of Episode 1, and now episode 2 is length. At what I'd consider to be a normal pace, the game took me nearly 6.5 hours to complete, and there's certainly replay value. The length of the game certainly is justified considering it's price, and the content makes it all the more worth while. The thing that really elevates Episode 2 is its fantastic character driven moments and its story. You'll often be on the edge of your seat wondering what is going to happen next. There are also some surprises (some not so pleasant) that occur, especially the ending sequence. In addition, several of the series's questions are answered for us, so if you are a fan, you should feel obligated to check this out!
The difficulty of the game has also been bumped up thanks to some lethal new enemies called hunters. The combine AI seems to have been given a boost, though for elite soldiers they still seem fairly unintelligent. While there are a few specific sequences that are very difficult, the game is still relatively easy overall, even on the hard difficulty setting. The weapons (with one minor new addition) are all the same as they were in Ep. 1, which to me is relatively disappointing because they are so inaccurate and unrealistic. It's a pretty minor flaw though, so don't let it deter you from purchasing the game. In my opinion, this is the most polished and engaging game in the Half Life series so far, it really draws you in! I'd even go so far as to say its the most polished game I'VE EVER PLAYED, but perhaps, being episodic, it's had an unfair advantage. That said, it is fairly similar to it's predecessors, it just does everything much much better than they do.