My sense is that this term, and the focus on it as an element of strategy, is relatively new (like a year old). perhaps there's already an article on it that I haven't found. This article might want to be quite short, but it ought to be on the wiki. Here are some thoughts to get the ball rolling.
What is endgame control (EGC), and why is it important? Having the ability to choose to end the game is nice, but you don't really have "control" unless you can end it and win. So EGC is the ability to end the game when you want to while holding or grabbing the lead. Contrariewise, EGC involves preventing your opponent from doing the same.
Dominion games end on piles or province/colony, so decks have EGC when they allow you to gain a bunch of cards all at once - but again, you also need points. So gainers in general help, but trash-for-benefit cards like Remodel, Salvager, and Counterfeit help a lot because they provide gains that exceed your average buying power. Goons is very nice as well in providing points and gains.
EGC is nice even in the mid-game because it can force your opponent to buy victory cards defensively instead of building. In the end of the game, EGC allows you to come from behind or nail down your opponent's coffin when you're ahead. When the score is close, you often find the lead switching from turn to turn; in this case, EGC means you can pick up the lead without letting your opponent come back.
Most big-money strategies lack EGC because they have no gains. They have trouble ending the game while ahead, and they have even more difficulty coming from behind. It seems to me that this is one of the reasons while money strategies are generally weak even when they provide slightly more points/turn: if I'm behind with engine, a big turn might allow me to pick up the lead and end it. Doing this with most money strategies is hard. (Counterfeit and Wharf are notable BM enablers whose gains provide EGC; this is one of the reasons Counterfeit so good for BM.)
One of the subtleties of EGC is that it's not just about deck composition. It's also tactical. This is why you have to take time to calculate at the end of a game. Decisions you make at the beginning of your turn may determine whether you have enough gains to end it - or whether you have enough cash to take an insurmountable lead. In my games with Stef, I've found he's exceptional at this, and I think end-game tactics are part of what separate the 30-somethings from the 40+ players.
So far, what I've written doesn't really account for the fact that EGC should also mean the ability to prevent your opponent from ending on a win. To accomplish that, you basically have to stake out a lead and/or keep piles from getting too low.
Your thoughts?