Fallout 3 isn't Oblivion with guns. And I'll tell you why.
I didn't like Oblivion, wether it was the crap voice acting, the wholey pointless leveling system or the fact that you effectively spammed the same button, I don't know. But I just didn't like it. Fallout on the other hand, I love. There are too many things to say about why you'd love it, but I'll pick my favourite points about it.
The Karma system. Alot of games can't do the whole Good/Bad thing very well, but the entire Fallout series has managed it. It doesn't just make people shout out expletives or say nice things about you when you walk past, it actually does something. It affects the radio broadcasts about you, the way certain quests can be played, opens up dialouge, helps perks... theres more, but like I said this comment will probally be big enough by the time I'm done.
Next, V.A.T.S. It's an obvious choice, but it's nonetheless awesome. It evens up the odds in fights, makes your opponents exlpode in a lovely gory mess and allows you to kill your opponent anyway you want. The large array of weapons also allow you to view the kill in a series of snazzy ways. Oh yes. Snazzy.
The DLCs. Op: Anchorage, the first DLC made me jizz my pants with glee. Fallout isn't heavily combat based if you don't want it to be, so the fact that Anchorage just allowed you shoot everything in your sight without reprocussions rocked. The Pitt which is just comming up looks good, but thats all I can say. Broken Steel on the other hand extends the storyline, and litterally adds more of everything. Couldn't be better.