Having played through a massive chunk of the story I have to say the main missions are pretty much as described. Drive somewhere, kills people, drive back.
But the real variety comes from the side missions, racing, dating, socialising, police missions, fire missions, ambulances, drug trafficking, taxi fares, finding stunt jumps, hidden packages etc. Then there's the multiplayer.
None of this is a million miles away from previous games but I think the hype is because it's reaching that triple distilled state of perfection. They're tweaking the formula with every iteration and the game has achieved such a large scope and managed to fit so much in and done it very, very well that it's hard not be amazed at the game as a whole.
The only thing about the game I dislike is the long drives from the start of a mission to the actual meat and bones of it. Can get really annoying later on when you're certain to die 3 times before you figure out the right strategy
But even then the fact you can just read a text message to instantly restart a mission is a vast improvement on the GTAs of old where you had to drive all the way back to the character who gave you it.