I've recently caught onto oGame, a game of galactic growth and conquest. At its heart you are a planetary leader rallying your troops to take to the stars and take what you can. Just like the other 15000 players on your server.
The mechanic is very RTS; You product metal, crystal and deuterium and use it to create buildings, defenses, and ships while researching new technologies that will, hopefully, give you an edge or erase someone else's edge against you. It's easy to pick up and anyone could play it. It really feels like a cross between Command and Conquer and Trade Wars. Not a bad distraction.
What gets interesting is the effect it is having on our highly ranked players; they are quitting in droves when they get bored, have their fleets decimated, etc. Why? You spend weeks, or months, building up your fleet only to lose it on an attack or have someone blow you away when your not looking (oh, did I mention the game runs 24x7 even if your not online?).
This is the type of game I refer to as "Infinite Grind", there are a lot of these, some more fun than others, but in the end they all suffer from the same problem; the player has to find things to do as the game offers nothing.
Give you another example from the MMO sphere; EverQuest. When I played you started in your home city and there was nothing for you to do. It was assumed you knew what to do. Great for a veteran, horrible for a new player.
Prefer FPS? Pick one, for the most part they all do this; you get weapons, ammo, armor, and go off to accomplish missions. Quake/Doom/Half-Life all have this effect - you know somewhere, once you have gotten all the weapons in the single player game, you are going to run into the obligatory cut-scene where you get capture/knocked-out/etc. and they strip you of all your weapons so you have to start again.
Why? Because your too powerful and the game needs to balance.
Infinite growth games are hard to balance as everyone grows and grows. In oGame the score to enter the "top 1500" moves up daily. Why? Those players are playing, building, and increasing their scores and increasing the gap between "Able to play" and "Dedicated Target". What fun is it being a target, unless you create a goal to be able to survive.
It's an interesting challenge, how do you create a game where people can build huge fleets and yet allow David to beat Goliath if he uses his head without ticking off Goliath?