The only thing it's too late for is getting the Scarab Lord mount (and title if you carry your character over to the BC servers when they come out). If you level too slowly to get the Argent Dawn Tabard in the pre-Naxx event, assuming they run the expansion launch events on the BC/WotLK Classic servers, you'll have a chance to obtain the Tabard of the Protector in the BC launch event and another crack at the Argent Dawn Tabard plus the unique Undead Slayer armor recolors (most notably the Judgment recolor for plate-wearers) in the WotLK launch event.
Playing on Bloodsail Buccaneers, the game is busy at all level brackets, and if you find a friendly guild, you shouldn't have too much trouble finding guildies willing to give you a hand for fast dungeon XP. Even LFG is pretty hopping even on off hours with people looking for dungeon groups ranging the gamut from Deadmines to Gnomeregan to Scarlet Monastery to UBRS. I assume the same is true of the higher-population PvE and PvP servers.