I have about 100 hours on M20J, but am a little rusty as have not flown one for a couple of years. Things I remember:
1) Prop clearance when taxying is minimal: take GREAT care on bumpy grass taxiways.
2) View over the nose on flare can be a little limited (relatively high cockpit coaming)
3) You have to sit fairly close to the control column to reach the (non adjustable) rudder pedals unless you have very long legs. This can leave you with the ability ONLY JUST to do full and free.
4) You need to plan your slowing down for the circuit well in advance to avoid either shock cooling or screaming in at 150 kts. To have to drop the gear to get the speed back quickly is, in my view, evidence of being behind the aircraft and poor planning.
5) All the comments so far regarding the landing technique are spot on. 5kts fast and you will float another 200m or more!
6) Make sure the baggage hatch is closed and locked before you depart. When it opens in flight it upsets the airflow over the rudder (not dangerously so) and makes you jump!!
With all this, however, quite the best touring aircraft I have ever flown. Solid and stable and fast, if a little snug in terms of shoulder room for two burly blokes. Visual nav was more difficult than radio nav because of the speed and the relatively poor downward visibility, but if I had the money and couldn't afford a twin then that's the aircraft I would buy.