1.d4 Nc6!?

That's because it isn't good! It isn't horrible, but not good either. Black often plays ...c6 or ...c5 in d4 openings. The knight blocks the c-pawn. I'd consider the Bogoljubow Defense dubious.

Not bad, as it looks, contrary to the most common opinions...
I got about 60% of success by blacks here, including 11 % draws, from my own 693 games here, that show ups on my Opening Explorer...

It transposes into the Nimzovich (2.e4), Chigorin (2.c4 d5) , or Mikenas (2.d5). It's playable, but as someone else already said, black shouldn't block the c pawn.

almost everyone plays dubious moves to try to get their opponents out of book. i think your best chance of suprising your opponent is to just play a good opening
====================
The good, the bad, and the ugly...
It is just an illusion, to think, that 1...Nc6 is a bad one response to any first move by whites...
it is just another mind set ups by blacks to mask your plans, first, and to trap the whites into unknown roads to go, where it is so much to fear for not brave ones...

I do not mean, that 1...Nc6 is a best one move by blacks, but, practically and psyhologically, it is really nice response to takes a fight on...
black also blocks the c pawn in king's pawn openings. that argument is invalid. the pawn structure is not yet set so its not clear if the c-pawn needs to move. as said before play can transpose to a nimzowitsch or even a scotch when it's not clear to me why the c-pawn needs mobility.

In 1. e4 e5 openings, Black rarely plays a c-pawn advance early in the opening. If it transposes into the Nimzowitsch or Scotch, the c-pawn advance isn't used in the opening. It's good if it transposes. If it doesn't, Black might wan't to make a c-pawn advance. This move doesn't allow that. I never said the c-pawn NEEDS to move. I said it's a good move in most d4 opening.
In e4 openings, Black rarely plays a c-pawn advance. If it transposes into the Nimzowitsch or Scotch, the c-pawn advance isn't used in the opening. It's good if it transposes. If it doesn't, Black might wan't to make a c-pawn advance. This move doesn't allow that. I never said the c-pawn NEEDS to move. I said it's a good move in most d4 opening.

I agree. I think Black can choose better moves, but this move isn't bad either. It isn't what I would choose, but it's a pretty decent choice. A transposition to the Scotch Game could be useful as well.
Off the top of my head, is 1.d4 Nc6 2. c4 e5 any good? 3. dxe5 Nxe5 is like a Budapest Gambit (but without being a pawn down), while 3. d5 Nce7 looks like some kind of benoni. If white continues 4. d6, then it is it good, bad, or unclear?
what's up btw with everybody suggesting white should take on e5 after e4 Nc6 2. d4 e5? thats just helping black gettinf a comfy position. f4 creates holes which black can exploit while a move like Nf3 just helps black develop with d6. the correct way is just simply playing 3.d5 Nce7 4. Nf3 Ng6 5. h4 askibg a question what that knight is actually doing.if de was all that i would face i would dwitch back to the nimzowitzsch immediately
this may be good idea when white has only played: 1. d4 Nc6 2.d5 Since you have not yet committed to any pawn move. However after your move sequence e5 is already played and is thus a target since the knight cannot defend it. If d6 is needed then i do not believe for a second in black's chances since he can now consolidate with 5.c4 , as Bb4+ is no longer a threat. This basically means black has gone into a KID or old indian defense several tempi down, openings in which tempi most of the time play a major role for example in the KID bayonet attack. White's plan is clear: castle short and roll those queen side pawns forward, while black's plan is not at all clear to me: going for a kingside attack, as the pawn structure dictates? but for that he's already several tempi short.
Nb8 cannot be any good. If black wants to play for anything he needs to play Nce7, unless you can find some tactical way to make it work, which i doubt since you have no piece developed.
what's up btw with everybody suggesting white should take on e5 after e4 Nc6 2. d4 e5? thats just helping black gettinf a comfy position. f4 creates holes which black can exploit while a move like Nf3 just helps black develop with d6. the correct way is just simply playing 3.d5 Nce7 4. Nf3 Ng6 5. h4 askibg a question what that knight is actually doing.if de was all that i would face i would dwitch back to the nimzowitzsch immediately
i think i was the only one who said that regarsing the nimzowich. i would auggest it because it is more of an advantage than d5 thats all. if you are happy with that position as black then great...play the nimzowich
I won't since white has many good answer to it with 5. h4 being best imo, and if all else fails he can always transpose to a scotch which can hardly be called a success if the goal is to get somebody out of book
http://www.chessgames.com/perl/chessgame?gid=1760596
Is this a good opening? How would you respond to Nc6?