For me, PvP (especially back when I ran the ladder) was all about going into a battleground, solo, and seeing what happened. Many times I got stomped so hard that I barely knew what my name was, other times we'd have a great game. After the ladder went away, I PvP'd even more because I was using the gear in PvE content - this was back before resilience even existed - and I wore some of that gear all the way to 70, as you can see in my mismatched set above. Even in BC, when resilience first took off and arenas were introduced, I often PvP'd to supplement my PvE gearing, or even to replace it on alts that didn't raid. But the more stratified PvP and PvE became, and the more gearing intensive PvP became, the harder it became to even do things like random BG's without first acquiring a full set of PvP gear. The barriers got taller and taller, and I was less and less interested in jumping over them.
I understand why this all happened - dedicated PvPers wanted a separate experience, free from the need to PvE at all. PvE players didn't like that sometimes the best route to getting gear was to run battlegrounds. Even now, with some 550 PvP items dropping from the Celestials, people are upset that there are PvP items that are better for them than PvE items. But for me, the solo PvPer, arenas and the stratification of WoW's PvP and PvE games became too great for me to keep participating. Even now that I'm PvPing more often, I'm only doing so at the end of the expansion, when I could buy a set of good PvP gear for justice points so I don't have to go into BG's without any PvP power and get steamrolled.