I learnt photography with a F3 (not HP), so I have a soft spot for it. It is a rock-solid unbreakable chunk of photographic precision.
My favourite features/thingies:
a) Metering pattern. I use it as a fat spot meter.
b) 100% viewfinder.
c) AE exposure that works flawlessly with negative EVs.
d) Mirror lockup.
e) Policemen, security guards and busybodies tend to associate it automatically with 'press'. I have gotten access to places I shouldn't have just by having it hanging from my neck. Something about a black blob of metal with 'NIKON' in white letters...
My not so favourite features:
a) +/- over/underexposure signals. You do not know how much you are over/underexposing, unlike a metering needle. If I want to place a shadow in Zone III, I must first place it in zone V with the meter, to fix exposure, and then close aperture or adjust shutter speed. A metering needle would let me fix exposure 2 stops under in one step.
b) The lcd panel is unreadable in low light, and the red button is not a high point of ergonomics/design/function.
c) The AE lock button is quite unresponsive. I am never sure if I pressed it enough, so I tend to press too hard, and this affects the natural flow of handling of the camera, making it stiff and mechanical.
d) Mirror slap/shutter. KA-CHACK! I remember shooting in a funeral that was held in our parlament. Everybody was silent, and the shutter sounded like rifle fire in the burning chapel. I finally went with mirror lockup (composing before locking the mirror!) in order to tame it a little.
e) Not true spot meter.
f) No shutter priority. It would be nice for shooting rock concerts, with changing light by the second, low overall light, and the slow speeds associated.
g) A bit too big. Takes a lot of space in my backpack; right now I use it for planned shoots, but my carrying camera right now is a Canonet QL-L.