The basic draw of the old world stuff for me is, I think, the story behind everything. I'm one of those gamers that (on my first toon anyway) read every quest description, marveled at the zones and the lore, and read up on a great deal of stuff. This is why I'm now leveling a Horde character--to see what I missed story-wise. When I play other RPGs or shooters I really like to poke around and see what kind of interesting nuggets of backstory I can find here and there.
For example--Half-Life, where in the various games you can see the G-man (and a lot more) every once in a while if you explore a lot. (My favorite shooter series, by the way.)
The thing about WoW lore is that it's not that great--B+ level fantasy, I'd say. It's got some great sub-plots (Tiron Fordring and the Ashbringer) and some lousy ones (Varian Wrynn), but the ability to be there and watch it all happen is really cool--it makes what would be a mediocre book into a near-transcendent experience. The first time I saw Illidan was a feeling of overwhelming excitement.
Of course, that feeling is likely easily exhausted--but as gamers cycle through WoW and new raids keep coming out, there's going to be a expanding set of old raids with a growing population of gamers that haven't seen any of it. I think that if people like me and Klepsacovic
can make a hobby out of old raid exploring with basically no rewards except bling, there must be a viable sub-market out there.