I hate the engine starting procedure. I really wish these engines started just like cars, boats, motorcycles, jet skis, etc. No priming, no hot start concerns, no accidental flooding, no crank, crank, crank, crank... then
Bang!
I hate preflighting on my knees. The older I get, the more I hate this part.
I hate my Garmin 430, but it is what it is. I can't afford an upgrade and I do like what it can do. I just hate, hate, hate the user interface.
I hate the fact that my plane is governed by certification rules and I can't just make simple modifications, or improvements as I see fit as well as be able to use better and cheaper experimental products.
I hate that it is just about as old as I am and just like me, seems to have problems more frequently than I would like.
I hate worrying about whether or not my Lycoming is going to produce metal on the next filter cut, or a crack in the case at any given examination.
Other than that, life is good!
As you can see, most of my beefs are with the general antiquity of my plane and it's tech.