Each new single player campaign will come with upgrades and changes to the multiplayer. The main difference here is that instead of giving you a tacked-on extension of the single-player experience as you often find in an RTS expansion, each title comes with a fully realized campaign.
Chris explained that each of the three installments would play a bit differently as well, with different meta-game types in each. The initial campaign would feature Terran Jim Raynor, with a meta game that involves upgrading technologies. The second focuses on the Zerg, with a meta-element involving diplomacy. I mentioned Star Control, and Chris said that that sort of gameplay element had been discussed.
More through the link.