I played World of Warcraft for the first time last summer, when the game was about four years old. I played City of Heroes a few months later. Despite the fact that these games were about the same age, my initial experiences with them was radically different.
My first WoW character was a Night Elf. I think I played for about two hours before I saw a single other player, which is pretty amazing in a game with umpteen million subscribers. Human contact was exceptionally rare in the first ten levels or so. WoW has a massive user base, but a vast majority of them are leveling characters or raiding in the end-game (as if this game had an end) instances. There is a saying among the hard-core WoW players that "the game begins at 80". (Or whatever the level cap is up to these days.) A good bit of the content is back-loaded, and you must play for days or weeks to get to the richest parts of the game. You earn your way into the fun content. I found I needed to hit level ten or so (which can take a few hours) before I could even get a sense of whether or not I was going to enjoy playing a particular character class.
In City of Heroes, I saw other players right away. The newbie zone was a lag-inducing crowd. (Even ignoring the fact that the initial spawn zone is also the hang-out zone for high-level characters.) People are constantly creating new superheroes, and the content for the first five levels is some of the highest traffic areas of the game. The game is front-loaded with content, so often the best stuff is early in the progression, and the later game is a bit of a grind.
Champions Online takes this front-loaded approach to fun even further. You can blow through the tutorial zone in well under half an hour once you know what you're doing. After that you get your travel power, which will let you fly, tunnel, swing, or leap around the world. This is roughly equivalent to getting your mount in WoW, and you get it in the first hour of the game. You get to start kicking butts and feeling like a superhero right away.
For me the meat of the game - the content - consists of cool new stuff. New powers to wield, monsters to fight, places to go, instances to visit, and game modes to play. Like any greedy gamer, I'm always looking forward to the next content cookie.
I can't speak for all players, but it seems like the old-school MMO games made you grind through the levels to get to the fun stuff. The newer ones - and especially the superhero ones - give you the fun up front and let you experiment with different character concepts and power sets. If you find one you really love, then you can play onward to the more repetitive sections of the game.