d4 tends to lead to blocked center, where both parties often end up pursuing their own side of the board. This blocked center limits the options for both players. Some players find this comforting as there are fewer surprises from the opponent. Some however find this restricting as this also limits their offensive options.
e4 is more flexible. e4 may lead to any kind of situation depending on the players. This allows the players to adopt a wide range of strategies. e4 can lead to violent and complicated positions, or simple positions, or blocked center similar to the d4 openings.
So is d4 better than e4? I don't think so. I think that with e4 there is a greater chance to win as there are more opportunities for both parties to make mistakes.
I agree with that. I think e4 is far more dynamic, and also it's best for leaning to deal with complicated positions.
I guess a player should start learning the queen's pawn opening only if he finds a tranquil game more suitable, although there are some complications at that opening too, of course.
Na5 hangs the e5 pawn, which is why it is never played. After 5...d6 a4 white should be better, which is why b5 is usually delayed a few moves.
We'll have to let Taimanov know that...
6.Nxe5 is a rare move that is well-enough met by 6...Nxb3 7.axb3 Qe7 getting the pawn back. Maybe White has a slight pull there but it certainly doesn't 'hang the e5-pawn' as you claim.