The Kingdom of Gilneas is beyond that wall. Being a lore noob, I dont have much information beyond that, but I would think if they ever have a Maelstrom expansion, it would be a good time to incorporate the rest of that peninsula. Even after the horde has taken over Silverpine, they could just say that that wall was what kept them safe from it.

There's a colony of humans who shut themselves off from the rest of Lordaeron when the plague started up. No one knows what's happened to them since.

On the other side of the wall is the kingdom of Gilneas. It's a human nation ruled over by Genn Greymane. He had the Greymane Wall constructed after the Second War in attempt to cut Gilneas off from "other people's troubles". No one has been allowed in since. (Not even the refugees camping at the wall's base in WoW.) You can read more about it here:
http://www.wowwiki.com/Gilneas

Personally, I'm curious about what has been going on in Gilneas since that wall was constructed. We may never know.

Like the others said, there's a whole human nation behind that wall. Gilneas. The people of Gilneas are douchebags who bailed on the Alliance during the second war (warcraft 2). The wall itself I think came up around the time of the third war (warcraft 3) or perhaps before and no-one's heard from them since (although I find it hard to believe that none of the flight masters were tempted to take a peek).

Considering their proximity to Silverpine it's often speculated they're all worgen now, but the truth is they're probably just living as they always did in their little bubble of 'paradise'.

As was mentioned Gilneas isnt the only human city on the west peninsula of Eastern Kingdoms. There's Kul Tiras as well...

There's nothing actually behind it ingame, besides a little bit of empty grassy area, but there's plenty of lore about it as mentioned in the other comments.

@Phun:
Given the nature of the Azeroth, I very much doubt things are going swimmingly on the other side of that wall. Whenever the gates open, I imagine we're going to find a place that's been absolutely ravaged by something or other. Naga and murloc invasions, dragons, corrupting poisons and magics, otherdimensional problems, teleporting mages, or even the worgen curse. Heck, maybe they just starved to death.

In any event, I can't imagine the gates opening to a happy country of idyllic beauty, and King Greymane just waving cheerfully at the dying pilgrims on his doorstep.