Hey @ slapin! Welcome!
To further elucidate on your inquiries:
1) T3D has AI classes built in, but the driving logic is not included by default. Duion suggested a good option with Bad Behavior, which lets you easily put together AI logic trees.
2) It has Flying and Wheeled vehicles out of the box. Doing stuff like AI for them, traffic simulations, or tweaking them to have the particular flavor of physics you need will require some work.
3) IK isn't currently in, but is on the TODO for 4.0. As J0linar mentioned, currently the bone limit is a bit under 100, but that'll be getting a nudge in the near future for 4.0 as well. As Chris mentioned, T3D has the Recast navmesh for navigation, so that should be a good foundation for your crowd sim, as it lets AI find their way quickly and efficiently.
Further, there's an Entity/Component system in currently, but it's still an experimental work in progress. I'm currently working on it to finalize it and it'll be rolled in officially in the near future for 4.0.
As for extensibility, it's definitely very extendable. Adding in new stuff, such as additional libraries for behaviors, creating new game objects and the like, are all fairly simple.
If there's anything in particular you need help with, feel free to ask, I'm sure we'll be able to at least get things rolling in the right direction