Most of the people who rate this poorly are just scraping the bottom of the barrel for an excuse to hate it.
The league and ladder system is well designed and implemented. Most Diamonds, and all Masters/Grand Masters, get there through tons of hard work. It's nice to be immediately recognizable as a strong player. Ladder is mostly cheese until high level play, but that's the fault of the players, not blizzard. People are lazy and want a trick to win games. The ladder system does what it's supposed to. If you go on a win streak you keep getting better and better opponents until you lose, which means that it's always a challenge.
Another aspect that people are hating on is the fact that they didn't get a Zerg campaign in Wings of Liberty..... So what? It's a complete game with a huge number of campaign missions, about as many as Starcraft 1 had with all three races. It also has the most balanced multiplayer of any rts game save Sc1, and it's more dynamic than it's predecessor. There's more variety in play styles than before and micromanagement is much more intuitive, yet more challenging in the right ways. It's less about fighting the twelve unit-select cap, or manually sending your workers to mine, and more about positioning, using abilities effectively, tactics, strategy.
Furthermore if the three games beef is money-related. Then you should have a problem with any other game out there that has expansions. Because that's what Heart of The Swarm will be. A full expansion. Not just ten more campaign missions for $60.