Why is 2. d5 Considered Weak?
Keep in mind that not often played doesn't always mean weak. There's nothing wrong with 2.d5, in fact Houdini actually considers it the best move for White. That being said, the variations that come out of it aren't as commonplace as Nc3 or Nf3 so if you want a comfortable position to play mainlines in, (and one where the position is basically symmetrical) d5 isn't a good choice... If you want an interesting position that you probably have never seen and usually means both players are relying on their improvisation, here's one example.
Tempo loss is a concern anytime you move multiple pawns in the opening to attack a piece, though I don't imagine tempo being a problem for White in the f4 variation, since black is going to have to make about as many pawn moves to develop his bishops or get White's center pawns off and the position is closed until Black makes those pawn moves.
In this position, why is it considered weak to play 2. d5 and aggressively attack black's night? My books show that the move is played approximately 13% of the time, with 2. Nf3 (53%) being the most frequent.