She is capable of keeping the rest of the Hyenas in line, which is no small task. She is also capable of thinking ahead, which is more than can be said for the rest of them.
Unfortunatly for the Hyenas, she lacked the direction to try and take over. I'm sure she could have done it on her own, if she'd felt the need to. For the most part, the Hyenas kept to themselves, away from the Lions.
Shenzi is a particularly fast thinker among the Hyenas. She exhibits an incredibly snappy wit, and is certainly game for a laugh, as long as it's at someone else's expense.