Well, the design has been through a few revisions. I'm not afraid to make breaking changes before version 1.0.0 (for example, I completely rewrote the level handling system to improve level editing performance by an order of magnitude).

The current status of the server is:

* Full support for Minecraft 1.6.4 networking (1.7.x networking is a WIP)

* Full support for editing Minecraft levels

* Implementations of all Minecraft data types (i.e. slots, metadata dictionaries, NBT, etc)

* Slightly buggy emulation of Minecraft physics

* Simple procedural terrain generation

Here's an incomplete list of things that aren't done:

* Lots of block interactions (like water emulation, or falling sand) were done, but were lost during the refactoring I mentioned above

* Redstone and all related logic

* Mob AI (work in progress, not very far along)

* More detailed world generation (missing ores, caves, biomes, villages)

Aside from the server, there's also a mature classic networking library, and a fairly complete client library (no rendering).

The project is more than a year old now and is built on top of code that's about three years old. It's a pretty big undertaking, and it's going to take a while to finish. It'll be done faster if I get more third party help, though!

Thanks for your detailed response. I see quite a few people are involved, and the project is a truly active one; I wish you luck. :)

One last inquiry: have you done any benchmarking / performance tests on your code? I imagine that since the library is thread-safe, it should in theory be able to handle significantly larger numbers of online users, etc.?

It's been a while since I've formally tested it, but it performs very well. I don't have hard numbers handy, but CPU usage and memory usage is both well-tuned (and better than vanilla/bukkit). I've tested with a few dozen concurrent players and it didn't have any trouble. All players reported smooth gameplay, far better than vanilla or bukkit.