This was discussed in the thread (not sure which place is best to reference so here are two):
or to see it all at once:
1. JavaDoc is insufficient, leading one to read the code to figure out how to use the class.
2. Deprecations are incomplete. The constructors that take String as an argument and the methods that take and/or return String should all be deprecated.
3. The allocation policy is too aggressive. With large tokens the resulting buffer can be over-allocated. A less aggressive algorithm would be better. In the thread, the Python example is good as it is computationally simple.
4. The parts of the code that currently use Token's deprecated methods can be upgraded now rather than waiting for 3.0. As it stands, filter chains that alternate between char and String are sub-optimal. Currently, it is used in core by Query classes. The rest are in contrib, mostly in analyzers.
5. Some internal optimizations can be done with regard to char allocation.
6. TokenStream has next() and next(Token), next() should be deprecated, so that reuse is maximized and descendant classes should be rewritten to over-ride next(Token)
7. Tokens are often stored as a String in a Term. It would be good to add constructors that took a Token. This would simplify the use of the two together.