I don't think it was really that serious a fight. Kind of like if a grade-school kid gets really mad at you and start flailing at you... sure you'll defend yourself and maybe slap him upside the head... but you're not going to actually fight. Then he sucker punches you when you're temporarily distracted.

Garrosh is right in the middle of his lifecycle as a warrior. Most soldiers/warriors have pretty definite stages. You have the new recruits who know they have no idea what is going on and either try to act tough, jump at their own shadows, or kind of accept things as they come. Then after they graduate their intro training and are no longer newbs they get a bit of confidence. As they get promoted or have some successful superficial actions they'll get like Garrosh and feel like they're hell on wheels... then after they've actually seen real warfare and have been in the blood and guts on the field, they start to become the grizzled veteran, like Thrall.

Often people who love war and fighting haven't really been in it. Veterans are a lot more focussed on getting the job done without getting themselves or anyone in their command killed.

I have no doubt Thrall could've shut down Garrosh if he actually considered him a threat. I'm not saying Garrosh is weak, but Thrall has probably forgotten more about warfare than Garrosh has learned.