If you listen to a song in 4/4 then 6/8 there is a huge difference. If you listen to a song in F and a song D phrygian there is a big difference. So my question is can we treat a time signature like a key, meaning writing songs in 4/4 all the time would be like writing everything in C all the time. There is alot of Time sigs and each one has it's own feel to it, though it takes getting used to times like 5/4 and 9/4 but they still sound great when done nicely. I'm currently working on a song in 9/4 and the rhythm is a bit wierd but sounds fantastic.

Yes! Indeed, the time signatures do each have their own feel and pacing. This is why a lot of waltzes use 3/4 time. Key, mode and time signature all have their individual effects on the piece.

If you listen to a song in 4/4 then 6/8 there is a huge difference. If you listen to a song in F and a song D phrygian there is a big difference. So my question is can we treat a time signature like a key, meaning writing songs in 4/4 all the time would be like writing everything in C all the time. There is alot of Time sigs and each one has it's own feel to it, though it takes getting used to times like 5/4 and 9/4 but they still sound great when done nicely. I'm currently working on a song in 9/4 and the rhythm is a bit wierd but sounds fantastic.

I actually did that in Airs de Daemon, where there were measures where I changed the time signature simply for the "feel" it gives. Using 7/8 -> 2/8, as opposed to 9/8, because the flow of music feels more like a pickup than a part of the phrase, and 5/8 -> 7/8 instead of 12/8 or 6/4 since it has a distinct pulse of 3-2 -> 3-2-2. It actually works out quite well.