@Artificial -

1) It can be argued that Thrall is just as wise and/or mature as being who have been in existence for thousands of years. But one thing that can't be disputed is that these immortal beings have something that Thrall doesn't: experience. They've been around for a long time, and they've seen (and learned) a lot. The amount of time Thrall has been around is but a blink of an eye to them.

2) Thrall is directly responsible for the Wrathgate Incident. Like it or not, as Warchief of the Horde, he is responsible for what his subordinates do under his watch. His lack of supervision and leadership allowed the coup of Undercity to happen (whether the coup was, in fact, secretly allowed or even encouraged by Sylvanas is a topic for another discussion.)

As for the topic of Garrosh being named Warchief, well... paging Artificial. Phone call for Artificial on the red courtesy phone from Saurfang the Elder, Vol'jin, Baine Bloodhoof, and the ghost of Cairne Bloodhoof on line one.

3) Interesting point. I never considered that.

@Jake - Did he? I always thought he was actually born on Draenor. If I remember correctly, the dialog in the novel "The Shattering" seemed to imply it. Oddly enough, I can't find anything solid anywhere on where he was born.