I always loved that. Yoda has a very good way of bringing out (in some ways) the worst in Luke, making him lose his temper and his patience, testing him before he even introduces himself. It's a fairly common trope in heroes' journeys.

And it's not even completely spoiled by the prequels. If you haven't seen them, sure, you're wondering what the hell is going on and why Luke is being accosted by a senile frog who talks backwards. If you have, you're thinking "oh crap, Yoda's been on his own here for 20 years, he's lost his marbles, Obi's sent Luke here for nothing and he's stuck here, the galaxy's boned." Not the same twist, but there's still a twist of sorts there.