I think the key there was the community. The feeling that we were working together and having someone there to just chat with while doing something so boring. I honestly didn't even stick with that group that long though. I think I might have played for about 2 months then.
Absolutely every MMOG on the planet eventually boils down to simply grinding away, wasting time. That's the whole point. The players are looking for an entertaining way to waste time, and the developers are looking to keep people playing month after month after month. They all get repetitive eventually.
What keeps an MMOG entertaining is
always going to be the MM bit - the other players.
Get a good guild in WoW, or a good corp in EVE, and you'll have the time of your life. Play it solo, and you'll get bored.
The last couple times I played, once I mostly ran missions and hunted rats. The other time I tried out trading. Both of those got old quick, but I didn't really have anyone to do them with either. So... yeah.
Mission running, ratting, trading, mining, exploration... It doesn't much matter, really. It can all be done solo or in a group. Maybe one will appeal more than another... But, when you get right down to it, it'll be more fun in a group than solo.