With enameled cast-iron, I don't bother to heat the pot before adding the oil. For something that's open-pored, like plain cast-iron or carbon steel (like an old-school crepe pan), it's better to preheat the pan. I can't see any chemical advantage to preheating enameled, as it's inert and non-porous. Of course everything DOES need to be properly hot before you add the meat.
I can take or leave letting my roast get to room temp when doing a braise. It's going to be in the oven/pot long enough that it's going to be cooked to 200+ degrees all the way through no matter what. The only possible advantage of letting it warm up on the counter is that it will take marginally less time to cook. And by marginally, I mean you probably won't even notice, on the scale of pot roast cooking time. For a steak or a rib roast where you want the middle warm without the outside being totally dry and charred, room temp helps a lot.
The only real tip I can give on pot roast is to use the right cut of meat. You want something from the shoulder, which usually means chuck. It has an excellent mix of rich meaty flavor, collagen/connective tissue (which makes that gelatin thats critical to the silky mouthfeel of pot roast and BBQ), and fat. You can also use something from the leg (shank) or tail, which also have loads of connective tissue, but they're usually a little low on actual meat. If you get the right cut of meat and cook it low and long enough to get that collagen rendered, it's pretty hard to mess up the rest of it.