What is the advantage of the QGD Orthodox Defense to the Slav?

I'm trying to find a good defence to 1 d4. I've tried King's Indian but find it to fiddly.

The Orthodox locks in the Queens bishop while the Slav doesn't . Yet the orthodox seems to be more popular. What is it's advantage?

There are many variations to each opening, leading to all sorts of play, so it's hard to write a few sentences that sum up the two openings. Also I want to add that the semi slav is a 3rd, completely separate opening.

But ok, you're just asking about QGD and slav.

In the most basic terms, I guess I'd say in the slav you're giving up the center with an early dxc, and in the queen's gambit you're holding on to d5.

In the QGD and semi slav you're usually needing to play c5 or e5 at some point in the first 15 moves, but in the slav it's more about piece play.

When I was new I thought the biggest difference was the light square bishop too... but for example in the tartakower QGD the light square bishop is perfectly happy and active on b7. So it's not really about that.

In general the QGD will lead to positions that are easier to understand than the slav and semi slav... that's not to say only beginners play it, tons of GMs and world champions play it too. So don't pick the slav or semi slav because you think that's what good players do, pick the opening that leads to middlegames you like. That's the whole point of the opening.

As for popularity, I don't know which is more popular. Maybe the QGD has more games in a database, but that's probably because it's been around longer. There are also many ways to transpose into these openings, so you can't just look at the 2nd move. For example a popular first move is 1...Nf6 which can become all sorts of openings. Black may often follow up with 2...e6 which still keeps options open for QGD, Nimzo, Slav, or QID.

Also I'm answering your question as if you're asking about QGD and slav as a whole... the orthodox QGD is a variation that is not very popular... but even more importantly, the orthodox structure is separate from the QGD orthodox variation, and that structure is something worth knowing about.

(go to the orthodox exchange section)

https://en.wikipedia.org/wiki/Pawn_structure#Queen's_Gambit_%E2%80%93_Orthodox_Exchange

"So it's not really about that."

The expert consensus is that the Slav advantages are a lot about the active and free light-squared bishop. Black can't choose a line in the QGD where the bishop is "happy and active", a major reason why the Slav is considered more aggressive also,. Also avoiding a minority attack.  

Advantages of the QGD are as you say better control of the centre, no real weak points and potentially a strong piece attack on the king side.  

Another mis-conception is that many have the idea that if the Bishop is outside the pawn chain, it's good.  That is not true at all!  It is still a bad bishop!  Bishops can be good or bad, active or inactive.  There is such a thing as Bad but Active, or Good but Inactive.  Good and Active or not synonymous and neither are Bad and Inactive.

In the French Defense, Exchange variation, 1.e4 e6 2.d4 d5 3.exd5 exd5, yes the Bishop on c8 is on an open diagonal, but it is still Black's "Bad Bishop".  The pawn that is locked on d5 is still blocking that Bishop on 2 diagonals and impedes the mobility of that Bishop.  Sure, it looks a lot worse in the advance variation, but it's still bad in the Exchange Variation.

The Light-Squared Bishop in the QGD, French, and other light square defenses can still play a vital defensive role.  Moves like Bb5 and Qa4 are often played in the Slav due to the weakening of the light squares on the Queenside since the Bishop can't come back to defend, and so if the Bishop is what is ultimately making your decision for you whether to play the Slav or QGD, that is a horrible mistake.

As mentioned in the post quoted, it's about which you prefer.  Surrendering the center, or keeping the d5 strongpoint and looking for a c5 or e5 break.  Don't let the Bishop be the decider.

This is correct. There are some crazy people going around this board that whenever all the experts and all the conventional wisdom says something, they seem to have an irresistible compulsion to say the exact opposite. 

The light-squared bishop and getting it outside the pawn chain is the main reason for the Slav. There's no "misconception" there at all. Stop being crazy. 

It's a good feeling to believe you have something to contribute to someone's knowledge. That you have some wise words of advice. But when it's contrived bull$hit that goes against what every expert has said from Alekhine to Tarrasch on up, then you're just objectively wrong. 

ThrillerFan don't project your misunderstandings onto others. I have already explained to you multiple times how black's bad bishop being inside the pawn chain is generally more serious than white's one. You're arguing now that the bad bishop is fine locked inside the pawn chain, meanwhile in that other thread you were arguing that it was a huge mistake for white to have it there when it looked like one of the main lines of the Semi-Slav where it's the main move. 

Slav defense is good, and Sam Shankland calls the semi-slav defense the best defense to 1.d4!

The advantage of the QGD is that you can transpose into it if white declines to go into the highly-regarded Nimzo-Indian defense. Slav defense does not have this advantage. However, slav defense has the disadvantage of having to deal with the exchange variation.

"In QG, there are three noticeable trends:

1. Black's 'problem piece' is the light-squared bishop on c8, since the natural development of the black kingside shuts it inside the pawn chain.

2. Black will normally search for counterplay by playing ...c7-c5.

3. White wants to remove the black d5-pawn in order to occupy the centre with pawns on d4 and e4.

How should Black respond? It depends on his interpretation of the relative importance of those three trends. For example, in the Semi-Slav [...] Black has concentrated on the first two factors and discarded the third.

The QGD prevents the third. By supporting d5 with the e6-pawn, Black maintains his control over the e4-square as 3. cxd5 can now be met by 3. ...exd5. Moreover, Black allows the dark-squared bishop to develop and thus begins immediately to prepare the development of his kingside and kingside castling. [...] Black inevitably has to offer a concession of his own: in this case, the passivity of the light-squared bishop at the start of the game,. Black's contention is that this is only a temporary feature that will quickly be rectified in the ensure middlegame". GM Matthew Sadler, "Queen's Gambit Declined"

Could be just a matter of what type of middlegame you are more comfortable with, since no opening will solve every problem for black.

You could also add in QGA too.  Some d4 players are irritated with QGA.

Yeah, to me it's more about the pawns... and like you said sometimes moving the bishop out before e6 (in various openings) is a big weakness due to the queenside light squares.

Here's a silly example, but I've had it in casual games a number of times.

And you know, to me it's a lot like when GMs give the scandinavian as an example of black losing time in the opening because Nc3 chases the queen... but white lost time first by capturing on d5 and brining black's queen to the center. So the net difference is zero. No one gained or lost time.

Same for the slav vs QGD bishop. If e6 "trapped" or "doomed" the bishop then why would it be a candidate for the most prolific opening ever, played by countless GMs and world champions? Obviously the bishop is fine, and chess isn't so simple that moving it to f5 or g4 before e6 is always better.