For years, I've been playing 1.d4 as White, and I can't say in honesty that I've done a lot of reading on the opening or anything; what I've learned, I've learned through playing the opening and by playing through the games of Garry Kasparov. I play 1.d4 with the hopes of going into the Queen's Gambit.
I don't know what 90% of the variations are called, but I know how to play them thanks to repeated use, post-game analysis, and GM games. There is an order to the familiarity of the openings, and I try to push games so that they transpose into a line I'm more familiar with, but my first 6-7 moves as White are usually the same in every game; the order simply changes. This is because I'm aiming for a pawn structure I'm familiar with; this gives me an innate advantage against anyone who is less familiar with the pawn structure.
The basic Queen's Gambit is simply 1.d4 d5 2.c4, but here it splits depending on whether Black chooses to take the c4 pawn. The simplest response to 2.dxc4 is 3.e4, but it's rife with peril; the c4 pawn is likely to be backed by 3...b5, forcing 4.Nc3 c6 5.a4 a6, tranpsposing into a line similar to the Main Line of the Slav Defense.
I don't know what an engine scores the above position as, but having encountered it dozens of times during those years when I exclusively played against The Chessmaster, I can say that it isn't a particularly good position for White.
Most people refuse to take the pawn, though, and it's not necessarily because they prefer the QGD over QGA; each has its own advantages and disadvantages. More often than not, it's because they sense that taking the pawn leads to an advantage for White. Which is true, in the sense that White takes control of the center, but White has a hard time doing anything about the monster of pawns that has accumulated on the queenside.
Interestingly, I see the prophylactic a3 played quite a lot in the QGD when I play it as Black; QGD is my response to 2.c4 when I'm facing against 1.d4, because I know the Queen's Gambit line very well. In one such game, I attempted to be friendly and told the person playing White that:
Bb4 is actually beneficial for White in the Queen's Gambit decline, so a3 prevents a series of moves that is ultimately beneficial for you. After Bb4, cxd5, exd5, then a3, Bxc3, bxc3, Nf6 or Ne7, followed by Rb1, and White is ready for c3-c4 and has claimed the half-open file for his Rook.
The advice got no response. In fairness, I was rated higher than my opponent, and I wasn't trying to be snide or rude. I was just offering insight into a line that I'm very, very familiar with:
...and Black has no clear way of moving forward. No doubt, there are ways to move forward, but nothing that is readily apparent. Heh. Disregard the last move for Black and the last move for White. I was in auto-pilot; Black needs to defend against Rxb7 and didn't do so in the above diagram. Defending against Rxb7 is easy, though, and has no real bearing on the overall position.
A few months ago, I analyzed one of my games and mentioned how much the moves Bb4, Bb5, Bg4, and Bg5 annoy me, and I said that they rarely accomplish anything and are easily dealt with. I still stand by this, at least in the ratings I'm at, because I don't often see these moves made for good reasons. They often seem as though they are played simply because the opponent isn't sure what to do with their Bishop besides either fianchettoing or attacking a Knight.
Bb4, Bb5, Bg4, and Bg5 can be tremendously useful, particularly if that Knight is the only defender of another piece or square, and well-timed move of Bg4 can totally derail an opponent's plans. But there are other squares that Bishops are great at. b4, b5, g4, g5, b2, b7, g7, and g2 are not the only good squares.
c4 is a powerful square for a light-square Bishop to hold, as is d3. Indeed, you'll see Bxc4 or Bd3 in all variations of 1.d4, because that is a great place for a Bishop to be. I particularly like Bd3, because the kingside castle leaves the h7 pawn very weak, and often only a Knight at f6--which can be pinned by Bg5 against Black's Queen.
Qc2 is another popular move for White in variations of 1.d4, and for very good reason. Bd3 doubles up the Bishop and Queen along the b1-h7 diagonal, and that's a powerful diagonal to control. Just as Qc7 appears in many of the Sicilian Lines, so does Qc2 appear in many of the QG/other 1.d4 lines.
The dark-square Bishop for White tends to be weak in most 1.d4 openings, because White's pawns often end up on dark squares. a3 and b4, with a Knight on c3 and Rook on c1, a pawn on d4 and e3 are all common following 1.d4, and a cursory glance shows that the dark-square Bishop is basically useless in such a position:
Black's position isn't terribly important in the above diagram; it's White that we're focused on, and White's position is one commonly seen after 1.d4 d5 2.c4 dxc4 3. Nc3... The dark-square Bishop basically has no way to get out and into the game; if at all possible, trading off the dark-square Bishop for a Knight or the opponent's dark-square Bishop is beneficial for White in most lines of 1.d4.
The light-square Bishop typically proves critical in 1.d4 openings, either to protect a future move of c3-c4 or just to pressure h7 or f7.
Conversely, Black's light-square Bishop tends to be underutilized in 1.d4 openings, because White's pawns usually land on squares that cannot be threatened by the light-square Bishop, and Black's pawns often end up on squares that block the light-square Bishop.
The main point of 1.d4 is Bxc4 and e4; 1.d4 is simply a means to an end, and the end is to get the light-square Bishop safely on c4, and to secure the e4 square safely against any moves of Black's pawns to d5.
1.d4 pawn structures are generally recognized by total decimation of the queenside and a glaring absence of play on the kingside. Be wary of Black's Qg5 in QGA and QGD lines! Due to the pawn structure, it can be difficult, if not altogether impossible, to get material over to the Kingside in order to defend against a subtle Kingside attack your opponent has been planning. Indeed, I did exactly this to someone who played the Queen's Gambit against me recently.
The vast majority of play during 1.d4 openings takes place on the queenside, and there is often nothing at all on the Kingside except the Kings and three pawns each. This creates a great deal of weaknesses that can be exploited.
Case in point:
Due to the nature of the Queen's Gambit, nearly all the play occured on the Queenside, as usual, but I saw the glaring weakness: White had no easy way to get anything over to the Kingside to protect against an attack. The computer blasted be for Bc5, but I still disagree, because that is what allowed my Queen to sureptitiously move to the Kingside and launch a quite unexpected attack against White's king. I had already moved White's Queen to a location where she couldn't zip across the board to do anything, and the only real defender White had was a Knight--who would be a free piece.
It was entirely because of my relatively extensive knowledge of the Queen's Gambit that I was able to win the above game. I knew the pawn structure, I knew the typical progressions, and I knew the weaknesses. So I capitalized on them, delivering a sudden attack that White never saw coming because the Kingside is usually neglected during the QGD. This isn't to say that I played the game perfectly--or even well--but it is to say that my knowledge of the opening allowed me to secure the win.
So be very careful in 1.d4 openings not to seal your pieces away from the Kingside. This is surprisingly easy to do, because the pawn on e3 prevents a great deal from transitioning to the Kingside for defense. And because, yes, most play and counterplay occurs on the Queenside in 1.d4 openings. It's not at all unlikely for the following position to be reached in 1.d4:
Afficionados of the opening will immediately recognize that as a typical QG pawn skeleton. The Queenside is often decimated and left bereft of pretty much everything. There is some room for variation: Black may have a Knight rather than a Bishop; Black's a pawn may be on a6 rather than a7; the Rooks may be reverse, with Black's on the c file and White's on the b file. The endgame here is slow, tedious, and not a great deal of fun, to be honest. If the Queens are still on the board, then this position is rampant with subtlety and tactical possibilities, so that's another matter altogether. But this position is a dead draw, and I'd advise you to accept/offer a draw before playing fifty moves in this position. It's not really very easy to screw up this endgame, after all; it's about as equal as any endgame could possibly be.
Well, I have to go do something now. I intended for this to be much longer and more useful, but hopfully something can still be gained from it.