Not many. The M112 V-6 is proving to be bulletproof, and so is the 98- version of the 4-matic system. The 722.6 tranny was sorted out early on, and is proving to be a tough tranny.
There are a few things that many modern cars seem to be struggling with, including the OBDII diagnostic system and the Mas Airflow Sensor. That said, the E320 overall is a pretty reliable car.
Recently Road and Track did a used car article on it, and they said the W210 E-Class was one of the most reliable cars they have EVER examined. Most owners were E320 that participated.
Dealers scored poorly, as us long standing MB nuts now well...
1998 C230 "Black Betty"