I killed a bunch of dudes in one area of the game pretty early on, and then left and did other stuff. It was probably 10+ hours worth of gameplay later that i came back, but the enemies were still lying there dead, and nothing had respawned. So i'm guessing the world is populated with X amount of enemies, and once you kill them that's it? I'm asking because i was wanting to find a spot and try and grind some levels, so i can hit 20 as soon as possible.

I was also wondering if there is any type of "New Game+" or any other reward for beating the game. It's be really cool to start the game over again at level 20, and retain all your perks, if not all your weapons as well... Is there at least an unlockable, extra difficulty level you get after beating the game or anything? Anything for beating the game at all?

No, there is no New Game+ and I'm not sure if enemies respawn, but there are random encounter enemies like the Regulators or Talon Company mercs depending on your karma.

"I'm asking because i was wanting to find a spot and try and grind some levels, so i can hit 20 as soon as possible."

It disturbs that you are interpreting that the game should be played in the style of an mmo.

Yeah, why do you WANT to grind? There is no need for it, and the game will be probably not as good if you do.

I don't think they respawn.  Doesn't Bethesda have certain rules in their games, like the world is what it is and however you change it, either by killing people, taking supplies from crates, or anything you do it stays that way.  So nothing respawns or comes back, you have to live with the consequences of your actions.

It takes a few days, but I'm pretty sure enemies repopulate in some areas.  I come back to spots all the time and there's new (and usually higher level) enemies standing right next to corpses of previous enemies.  Big Town usually has one Raider in it every time I fast travel there.  Try waiting three days before returning.

There are areas where enemies will constantly re-appear throughout the game, such as outside the Regulator HQ (two Giant Radscorpions every time you warp there). I would suggest just exploring and killing, killing, killing. The game doesn't make enemies respawn often at all, so it's better to level up through all combinations of discovering/lockpicking/killing.

"It disturbs that you are interpreting that the game should be played in the style of an mmo."

often when you load up a game most of the enemies that spawn outside of their own dungoens/instances are back to kill again.  DC is a very good area for this type of grinding, but i would suggest trying to complete the side quests to level faster.

"There are areas where enemies will constantly re-appear throughout the game, such as outside the Regulator HQ (two Giant Radscorpions every time you warp there). I would suggest just exploring and killing, killing, killing. The game doesn't make enemies respawn often at all, so it's better to level up through all combinations of discovering/lockpicking/killing."

I like the fact that dead bodies remain were you left them. I went looking for the Declaration of Independence and had a helper, but she died. I left her head on a wall light and her body on a bench. I came back 20 hours later and they were still there were I left them... cool.

Some stuff like raiders and talon mercs respawn, but after a while, they are just annoying and barely worth looting.

With the current level cap, there's no reason to actively seek out combat in the same damn areas, it's just better (and more fun) to get your exploran on and find some new locations and stuff to jack.

But if you've already found most of the stuff already and you've been at level 20 for ages when the cap get's bumped you could probably get into it, but at tat point you should just start anew, crank it down to easy as hell and get those remaining karma achievements/trophys

Just do sidequests, it'll happen naturally. Broken character advancement is broken.