Suddenly breaking it into three parts makes sense.
It's inspired. Back in Starcraft all the characters, backstory and plot for a given race had to be developed over 9 missions, plus a few extra in the other campaigns. Most events were big, and had to happen quickly. Starcraft 2 takes inspiration from the RPG genre - build it up slowly and get people immersed in the game.
About 5 missions in I found myself saying "Nice character development, average gameplay" - yet, I kept on playing with some degree of intensity. 2-3 mission later...still there, with the game's subtlety hooking me utterly. Starcraft 2 isn't just a game, it's a *story* - the character interactions and events are just as important as rolling out the siege tanks and firebats.
Gameplay-wise, not much has changed. Some of the research that used to be in-game has been 'spun off' into the RPG section on board the Hyperion (so the amount of repetitive mission research is reduced), so the game is that bit more focused on playing than chasing the tech tree. The game is great and plays well - the interface works well and controlling units is still very easy.
My complaints are minor. The music is nowhere near as good as Starcraft - on mission it gets lost in the background since it's far too subtle. On the Hyperion it's much better, where you get suitably Southern US music such as Elvis...but it's a shame the use of music is so inconsistent. Stability is also an issue - I've had the game crash twice, and required a system restart, and the mouse pointer doesn't work if you have your screen DPI turned up for your desktop (which is only really an issue if you game on a big TV like me, but I've seen no other game do that). But they're just enough to prevent this being a perfect 10.
This is a smooth RTS that isn't just about the campaign. It's about the journey of Raynor and his crew - and the way the campaign and story are woven together is borderline perfection. It even looks gorgeous consistently.
The game isn't cheap, certainly. But stuff it, complaining about the price when you're getting a top quality product is nit-picking - Wings of Liberty is worth the price you pay.