I've been using bullet for about a year and a half now in c++, java, and c# and I wanted to provide some general feedback on some more basic things. Before I start I just wanted to be clear that I'm a huge fan of the project and community and I plan on participating more in the coming years.
my observations (welcome feedback)
1- The pdf docs are nice but documentation seems lacking or inconsistent overall in the src.
2- Use of inner classes requires a more extensive work around for use in SWIG (try not to use them? move them and typedef them to their old location?)
3- default constructors for core classes (btRigidBody, btTypedConstraint, ...)
4- allow modification of btTypedConstraint rigidBodyA/B (at least if not currently participating the collision world)
5- allow modification of btBoxShape size (at least if not currently participating the collision world)
6- others like 4 and 5
7- git > svn (except for binary)
8- using CMake might cleanup the repo and reduce maintenance
9- Multithreaded could be built into each of the core projects BulletCollision, BulletDynamics, LinearMath as build flags (could put this in CMake also)