The two things that WoW really had going for it was the Raiding Endgame for gear, and the PvP.
WoW is much more then that, in terms of things both big and small. In the eyes of someone who has never played WoW before, there is MUCH more then that.
What makes WoW keep people that start playing are manyfold. First, and most important, WoW has powerful addiction mechanics. You always have unfinished quests, you always have someone willing to give you quests, and while everyone loves taking potshots at the crappy writing the quests offer, the writing is just a decoration of a set of objectives, and just that mechanic alone is a powerful tool to addict people. But most importantly with that system, and with what it requires to be functional, WoW's EXTREMELY well-polished in terms of progression curve. And this applies to the whole package. The number of quests you could be doing, the number you only really need to complete to level, and the fact quests always give you something beyond experience, even if it's just a sack of junky gear you'll sell for change, WoW has struck a really solid balance, and this is even more true with Cataclysm.
The size of the WoW is also humongous, and that means people who like to explore have a crap ton of exploring to do before they're really done. This is not something that can be overlooked. Even at level 10, you have multiple regions you can be leveling in, and this is not entirely race-locked. That's not something most MMOs can brag about. This is one of those things that truly gave Blizzard a leg up, they already had a giant world set up from their Warcraft series, they didn't have to spend time developing it, which is why most MMOs tend to have a much smaller world in comparison. (If you're thinking about TOR, chances are this will be true of TOR too.)
The battle system WoW offers keeps changing as you get new abilities and have to learn to use them in your current strategy. You can read a wiki and get the best build possible at every turn, but provided that you don't, trying things out with the new toys you get keeps things fresh. This is not something MMOs tend to get, and that's the core part of not making the grinding dull and repetitive. By the time it gets repetitive, you get a new thing you can work into your skill patterns and, hey, this is worth using more of, how can I best utilize it...? By the time you figure it out, the cycle restarts.
WoW is a lot of polish on top of well-designed addiction mechanics, truly nailing it as World of Warcrack. And even this post doesn't capture a fraction of what makes WoW so well-designed. Every single WoW clone or attempt at usurping WoW tends to fail because designers fail to grasp what makes the whole package of WoW so successful. Hate on it all you want, but it'll be a while still until an MMO overcomes WoW. Everyone still tries too hard to claw at its fame to try new things on their own. TOR might have a chance at shining next to it, but from what I get in this thread, TOR is going to be WoW with voiced quest texts. If they're lucky, they'll get the rest of the formula right, but I'm betting on them stumbling somewhere else along the way, as much as I love Bioware.