IMHO shocking documentation, very poor developer support via their Q&A stackoverflow clone and a somewhat flaky / buggy implementation (which could be mitigated by better documentation + developer support) or (open sourcing the framework) or (hiring someone to foster a developer community - none exists right now).
Maybe their paid support is better, but based on what is publicly available, I doubt it.
Incidentally, Titanium Developer (The build tool that compliments the framework) has been deprecated in favour of Titanium Studio, which is based on Aptana.
Many Titanium devs don't like Studio, and so have forked (the recently open sourced) Titanium Developer to remove some annoying anti-features put there by Titanium.
It's a shame; the Titanium tools are fundamentally good and are nearly there, but IMHO are frustrating to work with due to inadequate testing and an ineffective user feedback loop.
(Which BTW are a PITA on Titanium)