They're taking an incremental approach to releasing source code so as to prevent opportunists from copying their hard work and releasing full-featured clones of Nxt with no effort. In today's competitive market this seems like a fair and smart precaution. Bitcoin didn't need to do this because when it was released there was no market for it, and hence no one was trying to make a quick buck off of someone else's ideas. My understanding is that all or most of the new features will be open-sourced over the next 6 months, and by then Nxt will likely have a large enough market share for the network effect to give it a serious advantage over anyone trying to coy it, just like Bitcoin currently has over its clones.

The first Nxt source code release was on Jan 3rd, and it consisted of a slightly older version of the core components of Nxt with some purposefully injected security flaws to encourage the community to review the code and prevent adversaries from blindly copying it. Bounties were posted for the first people to discover these injected flaws. As far as I know, some great code review has been done, but no one has yet discovered the three purposeful flaws. Here's the active review thread if you're technically minded:

https://bitcointalk.org/index.php?topic=397183.0