Some technical details for Minecraft
Despite what some people say, modern Java really isn't that bad. It uses Just-in-time (JIT) compiling to achieve speeds comparable to a program written in a lower level language such as C++. It does incur a greater overhead due to type/bounds checks and garbage collection, but a well written program in any language needs to take care of these or similar problems.

The main simulator for Minecraft runs on a single thread and therefore core.  While potentially this makes poorer use of system resources (raw CPU speed is more advantageous than number of cores), it makes programming easier as handovers between regions can be a challenging task. Compare this to eg Second Life which runs on a distributed server system of thousands of cores. Currently all the worlds on a server run on the same thread as well.  As there are clearer boundaries here than within a world, handover is easier and as such each world could run within its own thread without much issue.

Minecraft creates a challenge for any server.  Whereas most multiplayer servers deal with static worlds, or at most a few dynamic entities per region, with limited interaction between regions, a large Minecraft server will potentially handle 1 billion blocks or more at any given time, along with hundreds of entities, and physics between potentially any neighboring pair of blocks, entities, or combination of blocks and entities.  Combine this with a dynamic lighting system with per block resolution and the ability to dynamically edit any block at any time, people should be going, wow, Minecraft is doing well.

Of course, with a system as complex as this there's bound to be some tweaks that can be done somewhere.  Switching languages is not likely one of them though.  In any case, Minecraft's full potential remains to be seen.