I actually switched to the Reti after learning basic opening play with e4. I added d4 to my repertoire initially to be better able to transpose between the two. I've tried c4 but never really liked the positions.
As for why I left e4, I prefer more closed positions with enough pawns on the board to facilitate maneuvers and breaks, and at my level most e4 games tend to lead to clearance in the center. Also, playing 1.Nf3 gives me the edge over opponents unfamiliar with the resulting key positions and themes.
Assuming everybody has started learning chess with e4-e5, what made you make the switch to the Queen's pawn?
And are you happy with your choice, what advantages have you seen?
I'm an e4 player and, while having spent a ton of time in opening theory against e4 defences, I'm now studying d4. My main goal is simply to learn new things and improve my general understanding of chess. Surely knowing how to play the QGD from white's perspective is going to be more beneficial than memorizing that 15th+ move of a sicilian variation.
I'm using Watson's repertoire book that promises to be strategical and avoiding the lines with monster theory.
My main aspiration would be to have a d4 repertoire that is based on understanding of general positional concepts, rather than pure memorization.
I say this because as an e4 player I'm frustrated by how much theory I have to deal with (and a mistake in the theory can cost you dearly). I've been playing chess only for 1 year and a half: going in a tournament against people who've been playing a line for 20 years (or even more!) I'm at a clear disadvantage.
So, d4 players, how does the theory workload compare with e4?
EDIT: "d4 players" means playing 1.d4 and 2.c4
Colle, London, Veresov etc. are "system players".