I think it would be a good idea to be able to bookmark in a thread so next time you go into the thread you can pick up from the spot you left off.
Meaning if you are reading a thread, and you have gone through 6 pages of replies you can bookmark reply #247, so the next day you enter that thread it will open up right at #247 so you can continue to read the rest of that tread.
If you bookmark another reply it will remove the first bookmark and create the new one.
What I do normaly is bookmark the thread on my browser, but it gets filled fast and some get forgotten about. The ones I come back to I never remember what post# I left off at and end up just stop reading the thread because some are many pages long.