As a personal preference only, both Diablo I and Diablo II have been amazing roller-coaster rides that lasted six months at best. Whether the in game cheating or simple boredom, there was always a finite feeling to the experience for me. How many times can one do a Baal run looking for a rune? Well I was taking this for granted, but good point I guess it needed to be said lol. Personally I don't understand spending money on anything that doesn't help you in-game. "Oh I got a little ragnaros" "can it hurt me" "no" "then who cares". Same thing with mounts, if it doesn't have a higher speed than regular mounts or do something that actually has a purpose in-game, I wouldn't spend money on it. On the other side of the argument, if blizzard was selling stuff that benefitted people in-game a lot of its player-base that earned their way would be upset, just as people are with RMAH.
I do believe that Diablo III is going to be an exponentially more fulfilling experience. Arenas, achievements, AH system, an entirely new difficulty level. The game without a doubt has more longevity than its respective predecessors. Having said that, the only game that would be considered a "wow killer" will be "Project Titan". I believe this to be the case for two reasons; It is a next gen MMO, and the simple fact that Diablo has never been competition to other MMOs. Diablo is an Action RPG and is in a category of its own, a genre also arguably made by Blizzard.
Even with Project Titan in mind, Blizzard has already stated that they anticipate a three to four million playerbase for years to come after the release of Titan. Even though this sounds absurd, please take a look at EverQuest, EverQuest II, Star Wars; etc. Come to diablo4gold.com, to enjoy more cheaper Diablo 3 gold and the fastest delivery. These games are ancient, have !@#$ty gameplay, outdated graphics, yet still manage to retain a collective audience of ablout 200k subscribers. These are MASSIVE numbers considering how many other MMOs are available out there today.
Lol with the annual pass if anything it's more of a WoW BOOSTER. Guaranteed subscriptions for a year, even if you subtract the value of D3 they're still getting $100 off that person at least, and it maintains their numbers of active subscribers for investors and just plain game statistics when comparing to other games.