That's the whole point - Luke was an impetuous "kid" at that point, with a minimal of Jedi training. Yoda told him he shouldn't go, but Luke didn't listen. Rushing off to face Vader at that point was totally in character, and he was lucky to escape w/ only a chopped-off hand.
Yeah, you likely got the "Special Editions", which means you missed out on 1 of the best things about Ep. 4. At the end of the Cantina scene, when Greedo confronts Han, you probably saw Han awkwardly dodge a shot from Greedo before returning fire. Super lame, I know, and not in character. In the original version, it ends like this:
Greedo: I've been looking forward to this for a long time.
Han: Yes, I'll bet you have...
**HAN BLOWS AWAY GREEDO**
No shot from Greedo, no dodge from Han. Just Han being a bad-ass.
Spoilered, since I'm not sure how much you know about the Emperor.
Prequel trilogy is decidedly less awesome than 4-6. They get better as they go. Ep. 1 is pretty shitty, and this is coming from a big SW fan.
I saw Empire the summer it was 1st released, but I don't remember if I knew or not before seeing it.