Oil is the blood of a transmission. This is how it lubricates itself and how it keeps all that heat under control. Running low on oil can cause your transmission to weld itself together. Things get cherry hot in there if there's no oil.
Synchronizer rings are one of the most common causes of a transmission not going into gear. A synchronizers job is to get the gear your going to shift to shift the same speed as the one your shifting from. They'll eventually be replaced because of the wear and tear they take.
Broken teeth on gears is caused by abuse to the transmission. People usually shift from 4th to 2nd giving it time to slow down but some "racers" try to shift from 5th to 2nd while still going the same speed. You can blow your engine, lose your clutch, and if you're lucky wont have to bring it to us. Remember no warranty on hard parts!
Just like the synchros, the hub and slider assembly are used for every shift you use the gear in. These are also susceptible to normal wear and tear but never try to force a gear in because it might be the last time you'll be able to get it out.
Bearings will cause lots of noise and if you hear a loud whine coming from the transmission the bearing might be on it's last legs. We usually replace bearings whenever a customer wants to "refresh" their transmission.
Bad seals will cause your transmission to lose oil and if you lose enough of it say goodbye to your transmission. A lot of people try putting in fluid once it's too late so whenever you see your transmission leaking (usually a red or brown fluid) check the fluid and bring it in for us to replace those pesky seals.