I have been playing the Starcraft 2: Wings of Liberty campaign fairly regularly since release, and I am almost done with it. I think I only have another mission or two left to go. I have been enjoying the campaign quite a bit. In terms of gameplay, the campaign is quite varied with well-designed missions. On the other hand, the campaign's story has felt very slow and directionless. So while the individual missions are fun, the campaign as a whole feels somewhat lacking.
The best part about the missions is that all of them play very differently from each other for the most part. None of the missions have been traditional "destroy the enemy's base" style missions. Instead, every mission has very unique victory conditions, optional objectives, and required strategies. Blizzard really explored the limits of mission design for an RTS, even more so than they did in Frozen Throne. MIssions such as intercepting trains, escorting evacuating colonists, and so on make for exciting missions that can really take advantage of the game's variety of units and tactics. While the game does have its number of "hold out against waves of enemies for x minutes" missions, it does keep them fresh by giving the player various different conditions or advantages.
Unfortunately, there are a couple of problems with the mission design. Almost all of the missions involve rushing the player through various mission objectives. For example, one mission might force the player to reach certain objectives before a competing enemy does while another mission might force the player to deal with explicit time limits. While this design is generally a good thing, since it makes the game tense and exciting, it does get old after a while. Furthermore, most missions are primarily designed to show off and take advantage a new unit that has just been unlocked. Between these two factors, it often means that the player rarely gets to take the time to experiment with units that were received in previous missions. There are a few units that I have only seriously used in the mission I got them from. Other units that don't have an affiliated mission are even worse off.
My biggest complaint about the game though is the pacing of the story. Overall, 20% of the campaign is serious main story advancement while the remaining 80% consists of side missions that don't directly advance things. Despite the length of the campaign, it actually feels like there are a lot fewer major story developments than in previous Starcraft/Warcraft campaigns. It also means that the story plays out very slowly, with some expositions at the beginning and most of the major developments weighted towards the end. While I liked the numerous new characters that were introduced to flesh out the Starcraft world, it feels like only a handful of them received significant development or screen-time. Stetman should have at least gotten conversations like Swann did, and a few more characters could have appeared inside missions.
Some special mention needs to be made of the Zeratul missions. While it was definitely a good idea to give the player a chance to take a break from the Terrans to enjoy some time with the Protoss, that entire story arc consisted of vague prophecies, serious plot and character retcons, anti-climatic introductions to long-awaited villains, and attempts to redeem established villains using the subtly of a wrecking ball. In other words, it contained all of my least favorite things in one short story arc. Can't Blizzard write a story without retconning their past works these days?
For the upcoming Zerg and Protoss campaigns, I hope that Blizzard continues the good work as far as mission design goes, but tries to add a bit more of the central story to the missions as a whole.