What I mean is that -- and this is obviously just my opinion -- a game should do its very best to make sure the player is always instantly, nearly and eventually involved. I know that sounds confusing, but let me explain before you start attempting to mail me lettuce so that I can pay a friend to throw it at me.
Being involved "instantly" means that moment-to-moment activities like combat should be involving and, this goes without saying, fun. If I'm playing a first-person shooter and the physicality of shooting someone just doesn't connect -- if it's too floaty or the bullets move through the environment in an unconvincing manner -- then I'll likely think said game is lackluster. Even if there's some really great progression or late-game features, it won't stop someone such as myself from playing for a few minutes before walking away with other matters on their mind.
The "nearly" part relates directly to MMOs. It's your character progression and the satisfaction gained from leveling, acquiring a new piece of equipment and basically chasing that next juicy reward. Obviously, having too little progression is very bad. On the other hand, there's definitely a point of diminishing returns where adding too many little carrots wastes development time and money, in addition to watering down all other rewards' importance.
"-when done well, word will spread beyond an established community and into the discussions of family and friends."
Which brings us to that "eventually" part of involvement. Every MMO has its milestones. Your first mount is a big occasion. A brand new zone to explore is like getting a trash bag of candy on Halloween when you were a kid. These moments need to be properly punctuated so they're memorable, so that a person will fervently tell a friend about their experience. In fact, each of these three facets are key because when done well, word will spread beyond an established community and into the discussions of family and friends.
Each and every one of these aspects -- instantly, nearly and eventually -- are really big chunks of an MMO. I'm well aware that there's a difference between saying they should be done correctly and actually doing so. I do know some simple choices that will improve the odds of players enjoying themselves in each aspect, however.
MMO combat is going two directions right now: shooter and classic. By classic, I mean games like EverQuest 2 and World of Warcraft. Shooter is fairly self-evident but there's a lot of variety to be found. Is the combat third-person or first? Realistic or futuristic/fantastical? Vehicles or infantry only?
Now, some act as if "classic" MMO combat is by default a bad thing. I don't think so. In fact, I think there's still plenty of room for improvement and growth, given the right circumstances. As someone who enjoys all sorts of different combat systems (from turn-based to FPS) I hope that developers continue to expand what it means to have "classic" combat. Star Wars: The Old Republic has an interesting cover system that may result in me playing a non-Force user class. Is it a shooter? Not at all -- and that's my point.