Gratz!
With you guys as my zone advisors I've already hit level 49, having pretty much finished Feralas and Tanaris with a few randoms sprinkled in. Where next dya think? Burning Steppes?
Un'Goro >Silithus - giant dinos, elementals, and lots of bugs; Twilight cultists and Cenarion rep; good for skinning and mining; Horde can get the Venomhide mount
Azshara - you'll have the place to yourself; demons, critters, giants, and shoreline mobs (naga, murlocs, etc.); good for all three gathering profs; changes completely with Cata
Plaguelands - mostly undead and Scarlet freaks; great for pallys and priests; Argent Dawn rep; the Pamela quest goes away with Cata
Searing Gorge >Burning Stepps - Dark Iron dwarves, spiders, dragons, and elementals; good for mining, not bad for skinning and herbalism; Thorium Brotherhood rep; BRD/BRS; the Chambermaid's enormous pillas!
Felwood >Winterspring - great for herbalism; critters, demons, and furbolgs; one of those Twilight/Burning cultist groups; Timbermaw rep; Alliance can get the Winterspring mount
There are also dragons, undead, giants, critters, yeti, and owlkin in Winterspring, but I haven't done much there. An empty firewater flask will start a quest chain that gets you a good +AP/+SP trinket, that's about the most I ever do.