I have played WoW and a bit of 4E, and I can see a number of similarities between the two. I'm not saying that they
are the same, mind you, but some aspects of 4e make me think of WoW:
1. The traditional roles are now officially referred as such. Sure, people have been calling fighters "tanks" for ages now, but only in 4E are they defined as "Defenders." Defender sounds a lot like tank to me. Granted, it really doesn't change much in the way the class functions; fighter go and kill things with sharp metal sticks, now and always. But the inclusion of Title carries with it a certain WoWism, if such a word exists.
2. Many of the class powers carry with them a WoW-like mechanic: Cool down times. Perhaps it was not intentional on the part of Wizards, but it is there. Encounter powers can only be used once per encounter, and are replenished after 5 minutes of rest, thus giving these abilities a 5 minute cool down. Many powers in WoW are similar, being useful only in a single fight (unless you are in a BG, or something really long).
3. Magic Items are very close to those in WoW in a number of ways. First off, any character can potentially craft magic items by learning the Ritual Caster feat; in WoW, you can pick up any two professions, regardless of class. Some of these professions are parallel to those in 4E: Alchemists make potions, enchanters can enchant varies items, etc.
In addition, there is the ability to disenchant a magic item into its component parts, producing a substance called Residuum. Using Residuum, you can, along with the proper ritual, you can make new magic items. With the exception of the Artificer in 3.5 Eberron, this has never happened in D&D (if it has, correct me if I'm wrong

). This is the biggest parallel to WoW in my mind; enchanters in WoW can disenchant magic items and make other items out of the shards and dust.
Now these are the things that stuck out immediatly in my mind. I don't play 4E anymore, so I may have missed some of them. I am not saying that 4E is Pnp WoW, I am just commenting on some of the similarities I, as a WoW, have noticed. Your opinion may vary, of course. But this might help explain why many people see a parallel betwen the two games.