I'm surprised WoW isn't on here. It gets a lot of hate for some reason, and Seige of Ogrimmar/Mists have been going on for awhile, but for a new player with the mindset of wanting to grind, there's endless things to do, and you can do them all solo.
Here's a little context from someone that played pretty hardcore from BC/Karazhan till the middle of WotLK, and has played on and off casually since then.
The first few rounds of leveling a character is fun (but I'll admit after you level a couple quests become pretty boring) - you don't just kill mobs endlessly, you level up by doing quests, and if you take the time to read the chat some of the quests are really well written.
Mostly talking about achievements, most of them are pointless but a lot of them give random rewards (pets/mounts/titles - gear is replaced pretty quickly in WoW so epeen revovles more around vanity items). For anyone with an addictive personality, achievements can be really fun.
There's loads of PVP to do. If all you want to do is PVP and not PVE, you can still get competitive (and in some cases better) gear and be able to focus entirely on that. PVP scales from battlegrounds with multiple people to rated battlegrounds that are more hardcore, to arenas that are 5vs5, 3vs3, and 2vs2.
It has a huge pet/pokemon type thing with hundreds of pets (ranging from really rare to super easy) to catch and level up. Again, there's a bunch of pet achievements that can be pretty fun.
There's loads and loads and loads of lore, and it entwines with every aspect of the game from questing to raiding. It's pretty cool if you're into that sorta thing.
If you want to get into endgame seriously, there's so many guilds/players out there it's generally pretty easy to get in as long as you aren't a terrible player. I won't get into endgame mechanics, but when I played seriously/hardcore(my guild was in the top 3 on the server) enjoyed it a lot. Achievements extend into raiding, and usually each set of raids/content has a set of achievements to do ranging from easy to massive pain in the ***, and in the end you get a mount, which is usually pretty rewarding. (Again with the vanity items and not gear brining in epeen).
If you don't want to or don't have the time to join a guild, they have a dungeon seeker thing for every available level of content. So you can seek and do current raids with a group that's matched by the system. Yes, sometimes people suck and the experience can suck, but in all the times I've gone it hasn't been that awful. Just for context, my 11 year old is able to do SoO and clear every boss in RaidFinder (and before anyone yells for letting my 11 year old play, he's pretty good dps wise and staying out of the fire wise, or else I wouldn't have let him go. In fact, he's generally better than most of the random DPS I've grouped with.).
All that being said, I found it's easier to burn out of wow than it was out of ffxi, just because there's always so much stuff to do and it can become pretty repetitive and overwhelming. However, had I stayed in competitive endgame instead of dropping out of my guild, I might've kept playing. Mists also sucked pretty bad, but the new ex-pac coming out in November looks really cool.
Sorry for the wall of text :3