Almost two months ago, an unofficial (completely reverse engineered) Vanilla WoW server called Nostalrius launched. I had a blast reliving the time I spent on the original game as a teenager, but had to wrest myself away after sinking a few too many days into it...
Last I checked, the server was averaging between 4000-6000 players concurrently, so I had quite the opposite experience from what the author was describing: for the first day or two after launch, it was very difficult to find mobs to kill at all, when you're sharing starting zones with literally hundreds of players! It got better over time, as people grinded at different rates, distributing their numbers more evenly, but even when I quit, it was pretty rare to find a zone without a decent number of players.
I recommend anyone with fond memories of Vanilla turned off by newer versions of WoW check it out. Or, maybe I shouldn't, having experienced first hand just how great of a time sink this 10 year old game can still be...
Nostalrius being based in France, and running zero Blizzard code on their servers, may give them some protection (IANAL).