I don't mean to say you're doing it wrong, but you kind of are. Don't take that the wrong way though, I'm not insinuating that you're stupid or anything, I mean on my first play-through of Fallout 3 (that was the first Bethesda game/RPG I ever played) I pretty much did
everything wrong for the first ~30 hours of game time. I was killed repeatedly by a raider with a Chinese pistol for God's sake!
Anyways, I'll start with your question of replayability, and the answer is yes, the game has a huge replayability factor. As CCNA mentioned
at least 3 playthroughs are required to see most of the Main Quest outcomes, but I would say 4 or even 5 playthroughs are required to see everything the the Main Quest has to offer. That said, the MQ doesn't disappoint -- it's filled with well written dialogue, allusions, satire, and morally gray choices that will get you thinking about which faction (if any) is
really the best for the Mojave.
I have done ~7 play-throughs of the game so far and I'm still enjoying it and continuing to start new play-throughs.
Why? you may ask; well it's because New Vegas is a Role-Playing Game, so I can play the game in a virtually unlimited number of ways and still have a different experience. That also goes for combat. You can play through the game using only melee weapons, or play through it using only energy weapons, or you could wear only light armor and take all of the perks that benefit light armor characters. You can even go through the whole game as a pacifist, never stabbing, shooting, or vaporizing anyone or anything.
Side comment: How do you get over melee being so silly and unrealistic. Granted lazors are laughable but there is a reason for the addage of bringing a knife to a gun fight ha ha
So that leads to my next point: that all of the weapon categories are balanced in Fallout: New Vegas. This means that a Melee Weapons based character can go through the game and be just as effective as a character who specializes in Energy Weapons. Just because Energy Weapons seem like they should completely outclass every other weapon class in the game, they don't. In fact, many argue that Energy Weapons are too
under powered when compared to Guns due to there being a greater variety of Guns skill weapons, ammo for guns being cheaper, and guns being far more durable while dealing just as much damage as their energy weapons counterpart.
So you might not want to bring a knife to a gun fight, but bringing a Super Sledge, Chainsaw, or Thermic Lance isn't a bad idea.