The very first MMORPG that I really put my time, heart and effort to playing was FFXI. For those who don't know what sort of game it was, it was (to put it as nicely as possible) an extremely party-oriented MMORPG. The truth is back then when I played, it is near impossible to solo after lvl 15, as all the experience points you need to get to lvl 75 can only be possibly obtained through party. You will not get any experience points from quests or anything like that. Sure there are quests in the game, but they don't give any experience points at all. So the only way is to grind with a party. Leveling in that game took me a very long time, but it had taught me everything I know about MMORPGs today.
The almost inflexible leveling system has its advantages: It forces you to party to get anywhere, and chances are, you will meet up with a lot of different people of different jobs and play with them. FFXI is a game where you actually need to watch the aggro on a monster, and so every player's level of skill and understanding of their job in the party becomes very obvious, and you get to know who the really good, and the bad players are. The parties last very long, that you end up chatting as you leveling, getting to know each other and getting along (or not). Some of them will end up being your friends, or enemies.
To be able to perform better at my job (monk) I deemed it necessary do some research on the forums, trying at improve my knowledge at my job. Just so you understand why, a monk is a DPS job, and the server I was on is littered with DPS job. To be able to make people remember you for the right reasons, you have to be better at your job than the others. I ended up logging on regularly, and when I am not in a party leveling, I will be somewhere farming for my expenses in-game (food, equipment & chocobo fees). It was a drag, but I am seldom bored, for I was part of a great Linkshell with a bunch of great guys and gals, who are not only helpful, but also have a wicked sense of humor that could make the grayest day look bright.
Parties are not only for experience points, but it helps build a rep for yourself as a solid team player. As exp parties usually last very long, members who are leaving are expected to give notice to their party at least 30 mins in advance. They are also expected to look for a replacement for their spot in the party, so that the party can continue to level after he/she leaves. Every member in the party is expected to know their roles in the party and perform to their peak. No one will leave the party if anyone is close to leveling. It is an unspoken rule that each and every member must bring their enough of their own food or other consumables to last the entire exp party. Going back to town to stock up once again after only 2 hours of leveling is almost unacceptable. Downtime was taboo. Experience chain #5* was king. Thinking back, parties I had back then seemed so professional compared to almost every other MMORPG I have played since.
Some of you might find that playing such a game is stressful and indeed it is. But the sense of accomplishment after every level makes that little stress seem so insignificant. One would feel good about himself/herself and you do feel that you are getting somewhere. The large amount of experience points needed between levels makes leveling an uphill task, but also makes it many times more satisfying when you get there. All the friends you have, all the hard work you done, and all the milestones you achieve will suck in you time and again. Even now, almost 3 years since I left the game, I feel tempted to go back, but the thought of going through all that grind makes me feel sick, and I wonder how did I manage to get through back then.
The answer, is because of my friends in the game. Although I hated the party oriented gameplay, I had the company of great friends, thanks to probably the most inflexible party system back then. As each and every one fell to the dark side (A.K.A World of Warcraft :D) and quit the game, it sapped my interest bit by bit eventually, and I quit the game as well.
Eventually, I did join some of them in WoW, but somehow I didn't enjoy it as much as FFXI. WoW is a better game overall, but still I prefer FFXI over it. Maybe I have grown more attached to FFXI than I would want to admit.
*For the interest of those who do not know what Experience chain i, I shall explain a little. When you kill a mob that is several levels higher than you, and you manage to kill another within like a certain time period, you will get obtain sort of a buff called EXP chain, and bonus experience points off it. The subsequent kill, made within the stipulated time (which is shorter than the previous one) will level up the EXP chain and give you even a bigger bonus on top of the base EXP. This can go up to Chain Five with the biggest EXP bonus.