Here is my experience from the White side:

1. Maybe i've been too influenced by Flear's updates without thinking much for myself. But the Blumenfeld seems reasonable for Black nowadays, and quite messy. Would prefer to avoid it as White if possible. 

2. Of course 2. Nf3 is a good move, but that also means White has to play the Queen's Indian (if he doesn't want a London/Torre). I think Black has MANY underrated resources in the Queen's Indian, much more so than the Nimzo (which is by far more narrow and forced). Preparing the White side of the QI is more time-consuming, but not necessarily more promising than facing the Nimzo.   

Let's not get carried away.  2 Nf3 is a perfectly sensible second move, flexible and applying pressure on the center of the board.  Is it possible that some players, rather than being afraid of a particular opening (I'm generally afraid of players, not the openings they play), prefer to play for a win in the middlegame rather than in the opening?  To suggest that 2 Nf3 inevitably leads to passive play is ludicrous.  At the same time, bluntness is not always a particularly becoming trait in a chess player.

This is a fantastic forum, but I think we frequently get a little too carried away with the first five or ten or fifteen moves in a chess game.  I'm not sure that equality guarantees any player in the world a draw or a minor disadvantage ensures one cannot win.  There's plenty of chess left on the board after 2 Nf3, and while the second player may not need to worry in the short term about holding off a strong onslaught, I'm always amused by the people who complain that they're bound to get a boring game.  Is it a boring game, or one where you didn't get to play your pet opening?  A boring game or one in which you are less comfortable?  Chess always holds some kind of interest for me; it's the lazy player who dismisses some positions as dull or uninteresting.  Some of the most interesting games I've played have been draws...

After 1.d4 d5 2.Nf3, my opponent has just shown me that he is not a free-swinging, hard-nosed fighter.  He's holding back.  He's being cautious and safe.  He's waiting.  Above all, as Willempie, alumbrado, and TopNotch have said, he's avoiding certain openings.  That's right, he has shown me that he is afraid of something.  Afraid of the Benko; afraid of the Albin-counter gambit, afraid of the Budapest, etc., etc.

I'm not saying that 2.Nf3 is a bad move.  It is a good move.  But, I am saying that white has signaled that he may not be a hard hitting aggressive animal.  He's like a boxer who cautiously goes to the center of the ring when the bell sounds and takes a defensive posture, but does not throw the first punch.  He waits.  He avoids.  The real killers will play 2.c4 or even 2.Bg5, like Mike Tyson in the old days, charging right across the ring directly at you as soon as the bell sounds. Avoiding nothing, just throwing punches with bad intentions.

- Lost Highway

I have just looked at some of my old games where I wanted to enter a Nimzo and got 2.Nc3 or 2.Nf3: after 10 moves in all games the same position: I (black) with d5 + e6, white with d4 + c3 - awfully (boring)! 2.Bg5 isn`t better at all.

"Is 2.Nf3 serious enough to make your opening decision depending from it?"
To answer this question, one must decide what kind of play one expects. If you want to play the objectively best moves, then my answer is no. I do not think 2.Nf3 is superior to 2.c4 and do not know anyone who thinks so.
At the other hand I want to create winning chances as Black, if White plays some dull setup. So I must take some risk, which automatically implies, that White's chances for an advantage also improve.
To make myself clear: it would be foolish to say, that the Dutch is superior to the Nimzo-Indian (or several other respectable Black defenses) because of 2.Nf3. Still I prefer the first. The reason can be found in a statement made by draughts ex WCh Harm Wiersma: one must not always play the best move, but sometimes the move that creates most chances.

2 Nf3 is not bad it is just that if you intend to play c4 it is a lot less flexible. Ok you avoid the Nimzo (which you also can do at move 3) and the budapest/Albin, but on the other hand you lose a lot of options ranging from the more agressive options from the QGD exchange to the KI. Not to mention most black players who play a Nimzo are willing to go into a QI anyway.

So of course you have to consider it, but it is less flexible than at move 3. Basically the only repertoire impacted by playing it second move are the ones playing Budapest and I suspect many of those will turn the tables on you by playing 2 .. b5 (I did that often after everybody started avoiding the Budapest).

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No, 2. Nf3 is a perfectly serious move and White in no way sacrifices his += birthright.  Of course, he does commit himself to certain systems and not others.  Just to state the obvious, he can no longer play a Saemisch or 4 Pawns Attack against the KID; nor a Switch Blade (American for "Flick Knife") against the Benoni; nor, of course, any system with the King's Knight on e2.