I actually thought of this before. A large (very large) server where almost everyone with Minecraft played. I thought the server could be organized like you were living an Earth life, just in Minecraft. Obviously, some cons would be lag, cheaters, bullies, and many others, but that is the same with all servers. The only thing is that it is really not needed. There are many other servers that many people are happy with. A mainstream, large server may not appeal to that many people. Not to mention, a server with (let's just say) 300,000+ slots would be incredibly expensive, and it would take much equipment. Honestly, I like the idea, but there is just too much to deal with.
this would kill 99% of current servers pretty much overnight, there is no way mojang could possibly staff it, and it would result in massive player dissatisfaction.
Imagine what the spawn area would look like after a week. After a month, new players would have to walk for a full day (earth day, not minecraft day) before they found a place to build their first house that isn't already claimed.