I think games should be fun to play - that's the idea. I'm quickly getting frustrated when I have to reload 20 times because a stupid jump requires some pixel-precise timing or stuff like that. Or if I have to study a game before I can play it.
They did a good job bringing people into the game with SC1, and I'm sure they do it again. Of course, already having experience with SC1 will surely help with SC2. And being able to set a difficulty level at last will definitely help not to get stuck at some point. Or bored of course. There should be a challenge, you are right with that, but any mission should allow a few little mistakes without leading right into disaster.
Maybe they even got setting up multiplayer matches figured out so that casual players can have fun amongst themselves and not just be cannon fodder for the pro players. I'll check this out too, but I'm mostly longing for the campaign. Oh, and I like it they made a 30 level all-Terran campaign which hopefully allows to go in depth. That was the weakness of SC1 - no Terran missions against fully geared-up Toss or Zerg.