Unless she got buffed recently (not sure, since I haven't been paying attention as much as I should) I don't think she's secretly powerful. Before the jungle changes, she was already in a tough spot, not doing anything others couldn't do better. I can't see how the changes would make her more powerful considering her lack of much sustain. The shield might help, but I don't think it would help very much, if at all. I might try her out again, I would love to go back to her as my main pick in jungle.
But anyway, to jungle with Diana is to farm until level 6. Her ganks are sub-par at best pre-6. You never want to dash in with your R unless you hit your Q first, that reset is what allows her to stick on her target. You never want to go in without activating your W, that shield does a surprising amount of damage. If you can, you'll want to build up your passive right before going in as well.
Really, DIana's power is mostly in the set up for the fight. Once she goes in, she's in. She's not getting out unscathed. If she is caught out of position, she's most likely dead.