For Alliance thinking of joining the red side:
The BE starting zone rates above other horde starting zones. But many hunters have gushed about the Tauren plains. (Personally: being able to belt peons with a whacking stick redeemed the orc/troll starting area for me.)
You are now expected to adopt a racial hatred of any preferred alliance race, gnomes and elves are the standard choices. Once you have done this construct a list of jokes and insults to do with cooking, eating, punting and the smell of these races to be used in conversation with your fellow horde.
Your new battle cry is simply ‘For the Horde’.
Red is a great colour for everything.
The best starting zone for cloth drops is the land of the Forsaken … because almost everything you have to smack is an undead humanoid.
Speaking of the Forsakan, The capital Undercity is so called because the city is actually underground … once you get there look for the way down below the surface – otherwise it can be a pretty dull town.
Gnome punting is considered a national horde pastime even though no one has actually worked out how to do it in game.
Blood Elf is the new least trusted race in the horde (used to be Forsaken that were the ‘clicky’ race), for some reason the BE appears in many alliance comics and wow movies, certainly they are quite fishy.
In the horde PVP mounts are considered pimp rides (outside of the rare raid drops.)
If an Alliance toon waves at you it should be considered a gesture of mockery, respond to this as you see fit. If an Alliance Paladin waves at you it should be consider a death threat – respond accordingly.
You are obligated to have the occasional ‘evil-me’ episode, you’re Horde now.
The Xroads (aka crossroads) is the home of the ‘new horde on the block’. Anyone who is lonely leveling should travel there to quest till around lvl 20 and experience Barrens Chat (although truth be told TRADE channel has been described as the ‘new barrens chat’ – but it can be pretty happening.)
The horde has ‘zepplins’ rather than trams … these are wonderful because toons can be mind controlled off them. Missing a zeppelin can be cause for much angst.
You can mount-up at the very nose tip of the Zepplin and jump off for a fast ride away.
The horde has capitals & outposts hundreds of feet up in the middle of nowhere: Thunder Buff, The Great Lift, 1000K Needles etc – Falling (or being MCed) from these areas to an untimely death is a regular hazard and seen as a great joke. Beware of duel offers from smiling priests when standing around in these areas.
The Thunder Buff pond is where many Alliance-fanatics (& other nutjobs) come as naked lvl 1 Taurens to yell insults and then drown themselves… its like lemmings. Festivals will usually result in a dozen or so bodies that can remain for weeks at a time.
The TB pond is also a good place to hang out in ‘angry-sea-lion’ form and meet fellow druids … when it is not full of corpses.
In TB the guild bank is actually that giant Totem pole about five feet outside the actual bank.
You have a new hazard to consider in neutral towns full of guards – beware of the NE shadowmelded on top of the vendor.
Orgrimmar is the horde capital (even with the Shatt city portals) – if you want to sell something, lock picking, enchants etc then go stand on the top of the Org bank and advertise in Trade.
Old school: As horde you should adopt a paranoid mentality of being outnumbered in hostile territory … travel in groups, be willing to ride to the rescue to requests made over general, and inform any high levels that request info on Alliance in the zone over Defence channel of any sightings or trouble. When you get to being a high level it doesn’t hurt to do the occasional zone patrol with mates to help out horde lowbies in known gank zones when you are bored. Great honour can be gloated about over General if you and a swarm of lowbies help pull down higher level gankers.