As a computer scientist, I've recently been fascinated by EVE Online. It is what I've always wanted an MMO to be.
EVE Online is an open-ended Massively Multiplayer Online Game (MMOG). Most other MMORPGs focus on a structured playing style with predictable outcomes and monotonous leveling. This seemingly innocent fact is why EVE is so different from almost all other MMORPGs, as the players have an incredible impact on how the game develops.
One could compare this to the difference between a playground, such as EVE, and a theme park, which would be the traditional MMOG. In a playground you have access to different kinds of toys and rides, and you are allowed to use your own imagination to figure out how to create games you enjoy. In a theme park all the rides have been created for you and are either good or bad by design. The playground clearly offers more freedom but it requires you to think and be an active participant, while the theme park has taken those responsibilities away from you and you can just go with the flow. As an interesting side-note, "theme park" style MMORPGs commonly develop lines, just like real world theme parks, as players wait for monster spawns, rare items, or quest requirements.
* One universe; there is only one instance. Every player on Earth (with the exception of China) that plays this game will be in the same world in this game. So far, I've heard the peak player count was nearly 40 thousand.
The EVE Online supercomputer cluster manages over 150 million database transactions per day on a 64-bit hardware architecture from IBM. The database servers don't use traditional hard drives but instead Solid State Disks (SSD) which are recognize as the worlds fastest storage devices with over 400,000 random I/Os per second. The recent world record of over 30,000 concurrent users on a single shard is a testament to the clusters capabilities and CCP is looking forward to support at least 50,000 concurrent users.
* When your ship blows up, it really blows up. It's gone. You can insure your ship, and get back a portion of the cost, but it's really gone. Looks like you'll have to mine some more minerals, and build another.
* Finite resources. Oh yeah, and not only do you have to mine more minerals for that ship; you have to fight other people for it too. The first lesson of economics is scarcity, and this game has a quarterly economic newsletter.
* Unlimited Options. You can do whatever you want. You can be a pirate, and blow up other people, and steal their stuff. Then sell their stuff for money. Or you can form up with some other people, build some starbases, and establish a sovereign state.