Starcraft 2 is the perfect sequel to the original starcraft, adding many more things while retaining the charm.
Let's start with multiplayer. First off, it is certainly not beginner friendly. I highly advise you play the original if you're just starting with the series, or at least get a few ai games under your belt. Blizzard has added many improvements. A new friend system, matchmaking, and portraits to brag about your conquests. The multiplayer is a blast, with up to 4v4 matches. Custom games are an added bonus as well, with a feature that shows popularity of each scenario.
New units and building upgrades have been added, giving us a lot more tactics while keeping the classic balance of the races.
Great game!