Do you like to play with others in a MMORPG? As you know, World of Warcraft and Rift use two very different models for cooperative PvE: In WoW a raid leader carefully selects who to take on a raid, because if he takes on an underperformer, he risks the whole venture to fail. Thus much of the "social" game of World of Warcraft consists of weeding out the underperformers, of separating the "cans" from the "cannots". This takes place everywhere, from guilds, to trade chat, to pickup groups. In Rift there is not such selection. People join groups by simply showing up and can even join other's pre-formed groups that aren't full.
Reading around the blogosphere about people's experiences while playing Rift, you will notice two things: Most players are extremely enthusiastic about Rift and how it handles playing together. And secondly, nobody is even mentioning the word "challenge". The sort of gameplay where you have to perfectly execute a boss strategy or wipe repeatedly not only doesn't appear to exist in Rift (yet?), but also nobody appears to be missing it. Instead Rift players are talking about the possibility of Rift being the fabled "WoW killer".
That opens up the question whether World of Warcraft, and especially Cataclysm, did it wrong. Certainly the most hardcore players are not only advancing at a reasonable pace, but they are also extremely happy that the majority of players is *NOT* advancing at the same pace. To feel elite, you not only need to succeed, you also need the majority of the rest to fail. Cataclysm certainly delivers on that account. But that catering to the leet comes at a huge price: Cataclysm has made World of Warcraft an even less social game. I was stunned when recently somebody made a snide remark of some of my gear "not cutting it in heroics" in a guild group! Our guild used not to be exclusive like that, but rather took extra care to include everybody. And there are a lot of people either quitting WoW in frustration, or quitting their current guild in favor of one advancing faster, in spite of the new guild perks system that was supposed to prevent guild-hopping.
At least for now, Rift's "you advance just by showing up" model appears to be wildly more popular and fun than World of Warcraft "show up, be judged on your gear and/or performance, and then get kicked" model of cooperative gameplay. Of course you can insert a Gevlonesque comment here about how of course the morons and slackers prefer a game in which they don't have to perform. But take that comment and remove all the negative judgement from it, and it becomes: "Most players prefer a game in which they don't have to constantly justify their performance". And that is a truth which Blizzard would do well to ponder. They might not get "killed", but losing a couple of millions of players to Rift, and then potentially SWTOR, certainly hurts. Given the difference in available budgets, it is remarkable how well "inclusive" Rift is doing against "exclusive" World of Warcraft.