An average expansion for a great game...
Act V: Act V is, when it comes to graphic details, the best act in the entire game. The characters on the town are ok and the monsters are quite different to the ones in previous acts, which is a good thing.
However it does not make sense that the monsters from Harrogath are stronger than the ones in hell. This game is called "Diablo", not "Baal" and I think Diablo should have been the final and strongest villain.
The new characters: The assassin and the druid are weaker that the original characters and fail to catch my interest. They could have done a lot better.
Runes, rune words and jewels: The best part of the expansion, they allow you to create new, more powerful items.
Charms: Some charms are really good, others (the vast majority) are crap. Don't waste your space on them unless they are extremely good.
Private stash: It could have had more space... but it's an improvement.
The expansion also adds character-based objects, which are a great boost, and lots of new items.
In other words, if you liked Diablo II the Expansion is worth buying, but it could have been better.