The massive bugginess is #1 what eventually killed my motivation for the game. (Plugin scene drama and Mojang EULA dickery being items 2 and 3).

It's very hard to enjoy the game when making builds over a certain size leads to all manner of graphical glitches and memory leaks and other weirdness that you need to be intimately familiar with the operation of the Java virtual machine in order to fix. A number of these bugs lead to world corruption - I remember back when 1.7 had its initial release, there was a certain configuration of redstone blocks and mine carts that would "corrupt the world state" and leave you with an unplayable world.