I'll third (or fourth or whatever) Riverglades - it's an incredibly strong and enjoyable zone. It's almost TOO good - the great layout and quest design makes it stick out a bit from vanilla zones; it feels almost like a WotLK zone, honestly.
Quick tip: if you're a leatherworker/skinner - farm the coyotes directly south of Wheeler's Grange in Riverglades; they're lvl 37-39, but drop Thick Leather at very high rates for the level. They're also pretty weak and, while there's a quest mob there that respawns quickly, there were never many people there to contest the spawns. It's an amazingly easy place to farm thick leather and make a ton of money for any skinner.
Because of all of this, I hit 46 on my PVE human hunter tonight - I'm having a blast. Got my mount and my dual spec already, as well as 5 Legacy Points that I dropped into the one that gives 5% health and mana regen on kills in the open world. I probably should slow down, as it's getting tough to find groups for dungeons - for most of my 40s, it's been more efficient to quest in good zones like Riverglades.