I've already talked about the virtual ghetto. Too strong an image, surely, but I'll stick with it for now.
I recently made level 58 in WoW, which is the level you need to be to get from legacy land to Outland, the new parts of the world accessible only with the WoW expansion, The Burning Crusade (TBC, as if you hadn't heard).
I've not got the expansion, but I have noticed that the items TBC players are all but throwing away when they do run back by Ironforge and my guild are incredibly good. I've recently gotten both an amulet (for free) that kicked the pants off of everything I've owned and a staff that increased my all-important "damage per second" rating by about two-thirds for a lower price than the guy could have gotten selling it to a vendor/NPC.
The key point is that both items are from Outland and both require level 58, obviously targeting a floor that coincides with the entry price for TBC. I'm not sure, if this hunch turns out right, that Blizzard considered the effect of TBC on those who don't upgrade. If so, I'd expect less of a quantum jump in the amount and power of these now lower-level items.
As a counterexample, right before scrounging up 5g for the staff, I spent 15g as part of a quest to get a key to Scholomance, a major dungeon in the "old" version. The conventional reason to pay so much to gain access to the dungeon is that it's an excellent place to find great equipment. Well, now the hand-me-downs from TBC gamers are beating the socks off of Scholomance gear, removing a good deal of the incentive to run the dungeon. I asked a buddy who has TBC how the equipment was, and he concurred with my assessment without any prompting. There's no reason to hang around in any place but Outland other than nostalgia, a few now much easier to kill bosses, or, well, that you haven't spent $40 to keep playing.
I have seen some places where monster drops (what you get when you kill them) seem to have changed considerably, and perhaps Blizzard plans to overhaul the dungeons in the old world. Until then, however, the divide between John Edward's Two Azeroths seems to be increasing.