I would be surprised at how popular 3.3.5 is relative to 2.4.3, but I'm not due to how bad the infrastructure, rates, no TBC - Blackrock, and Inattention towards Outland.
First of all, PvP is much different.
There would be no need of a "Lower" and "Upper" bracket here because fresh 70s will contribute during the fights. On Wrath, the fresh 70 will be a liability because gear still matters, but not as much as wotlk.
There is less Bursting. I played hundreds of AVs on both Wotlk and TBC. I never once had a game in TBC were the enemy team quickly raped the guards, ran into the tunnel ASAP to graveyard camp. The guards and bosses also require more players to kill, the boss dies slower, and it's more of a "battle" then bursting it down. The Gargetzan guards beat players on TBC, in WOTLK the players beat the guards no prob.
There is more Counter-play and Skill. You can't just burst a player down in seconds here like you can in 3.3.5. Burst still exist here, but it's more rare and mostly a team combo. The PvP is also not turtle speed, abilities have more "value" were choosing the correct ones at the correct time will win you the battle. This is not a expansion were you purchase or create a BiS Paladin + SM, and start mind-numbly press 4 buttons to win. You will be using most of your abilities, and be thinking what should I have done instead to win the battle instead of thinking "GG, they have BiS gear" or "Looks like they Burst us harder than we Burst them, GG".
The Classes are more balance except for Rogues and Druids. Paladins and DKs (not in TBC) are not the Melee Crown Jewels here. Rogues are a little stronger than other melee, but by no means hard counter the other melee. Resto Druids are also a little over the top, but not too insane. Ranged/Casting classes are very balanced here. In wotlk, Holy Paladins are like the Resto Druids in TBC. Overall TBC seemed more balanced in terms of class balance.
Overall, I don't understand the appeal of wotlk PvP. It's more toxic, gear-depending, bursty, less counter-play, skill. It's great if your lazy and just want to win in BiS gear as a Ret Paladin, the problem is so does everyone else. Other classes in WOTLK like a MM Hunter does require more skill tho.
This is what would separate a TBC - Blackrock from Blackrock. Blackrock is a great realm, but as a casual myself, the realm seems just to damn Hardcore.
Sadly, I can't tell you about the PvE because I only PvP.
For the questing experience, you can get a taste by doing them on your wotlk char and imagining this being 2.4.3 xD