Pre-BC there was little gear benefit from PvP. The best gear was incredibly hard to get (at least until 2.0 when PvP gear went to a simple honor grind with no ranks). But despite the effort, it was not the best PvP gear. Raids gave better gear for PvP.
"Raid or die." It was pretty simple, PvP gear was from raids, so people were forced into PvE to get gear for PvP. This wasn't much fun for people that just wanted to PvP and had to choose between sub-par gear or doing content they didn't enjoy.
It got fixed in TBC, sort of. Now PvPers can get all the gear they need from PvP. Great, right? Not exactly. The problem just got reversed.
PvP gear has few restrictions on who can wear it and doesn't require large raids or lockout timers. It's incredibly accessible. It is also very powerful, with S3 blowing away T4 for many classes. Unless you're in very high end content, PvP is going to give more powerful gear than PvE.
PvE people now have the choice that PvP people used to have: inferior gear or unwanted content. Raid or die didn't go away, it just got turned around.
14 minutes ago