Starcraft 2 is exactly that: a modernized version of the original. Gameplay is essentially identical, but everything that centers around that has changed: detailed stat pages, wonderful Battle.net friend systems, replays, new units, rebalanced races and of cours updated graphics. The beauty of the game lies in the incredible balance of the otherwise extremely varied races: every action has an equal and opposite reaction. It's just a question of picking the right actions and reactions, which makes this possibly the purest strategy-heavy PC game currently in existence.
Add to that the wonderfully varied single player campaign, built on a fun but campy story that takes you to a bevy of inter-spacial locations, and you've got a solid entry into the Starcraft series. People have critiqued the game for being split in three, but the truth is that each entry appears to be longer by itself than the full campaign of the original. Which means more Starcraft for us, which is never a bad thing!
This is a game that, as with the original, I'll be playing for years to come.
In a sentence:
Exactly what I hoped for in a Starcraft sequel, with a great single player campaign, fantastic multiplayer tools and gameplay that is as addicting as ever.