You have to look a bit further than that. WC3 has 4 races, WC2 had 2. Most of the units are new and the gameplay is improved, as well as the hero units. Those are quite big changes for an rts, if you compare it with other titles.
And then StarCraft 2. Did you take the time to download and watch the gameplay trailer? I reccomend it. In it, you could clearly see that Blizzard only had 2-3 of the same units per race, and that those old units had a new ability as well, although they only show the protoss. The concept is still the same, gameplay will be different, story will be different, and that's what making a sequel to a game should achieve.