Also, the Orcs on Azeroth seem like a genuinely bad race now, seeing as they can't blame everything on "I drunk da demon blood, thats why I killed half your race, we cool now?" and were seemingly easily corrupted and manipulated.

They had evil potential, and it was very VERY easily drawn out of them.

Tbh thrall seems to be the only good one, having no crimes or past sins.
No wonder he's trying to become the new guardian and leave the Horde in cataclysm, if I were him I'd get the hell away from them too.

I cannot believe you'd put those guys over Sargeras. I'd suggest reading some of the novels related to the Warcraft lore before judging him; he isn't as wimpy as you'd think. :P

And yes, Kil'jaeden thought the orcs destroyed the draenei so with his vengeance seemingly complete he thought he had no further use for the orcs and left them. Gul'dan was actually pissing in his pants when he found out that KJ had abandoned them and he now had a bloodthirsty society hell-bent on fighting something with nothing to fight and dwindling natural resources.

Also, it wasn't a case of Kil'jaeden not wanting to bring the orcs to Azeroth. He simply hadn't thought of it and when Sargeras thought of it way later, KJ didn't disobey (not that he could if he wanted to live.) Also keep in mind that it wasn't Sargeras himself in the sense that he was actually inside Medivh and came to Gul'dan as Medivh.

The Orcs may have been easily corrupted, but so are most races, heck the Draenei themselves are a mere remnant of another corrupted race. Everyone wants power. If Kil'jaeden appeared in front of King Wrynn and offered power enough to destroy the Horde, you better believe he and all his loyal humans would drink the blood.

I think one way to keep the threat of Sargeras alive is to recognize that he's playing a very, very, very long game. We lesser beings see him move a pawn one square and wonder what the point is. But Sargeras is planning thousands of moves ahead in a multidimensional chess game against the greatest powers in existance.