Essel,
The foregoing is good information... To add to it, light aircraft do not come with MEL's (probably because there is no requirement for one, and people are not willing to pay the cost to create it). So, what does a pilot do when an unserviceable item is found? It's up to the pilot to judge, which puts a lot of responsibility on that pilot. Yes, the regulations specify certain minimum equipment, and you must not depart without it. Unfortunately, those lists are not together, and can be hard to sum up. If you miss one, it's your fault.
So, what I do is consider the situation the other way around. If I crash the plane, and the absence of item "X" is in any way the cause, would I still be confident to explain to the authorities, and the insurance adjuster what I was doing flying the plane in that condition? How serious would the accident be, if the missing equipment were a cause?
If I depart for an intended day only flight knowing that the nav lights are U/S, I think that most of us would be okay with that. If I continue into night, and have a mid air, it's all different.
Stall warning system? I wouldn't! If you had any kind of landing mishap, that's probably what would get the blame, then you, for choosing to fly that way. I did knowingly test fly a Cessna 185 some time ago with a U/S stall warning horn, only to find afterward that the Type Certificate Data Sheet (the bible for that plane) specifically says that the horn must be functional. I would have never thought to look there for "MEL" information! To make matters worse, I had allowed someone else to load the plane for the test (too much trust) and I later found that the C of G at gross weight was 1" behind the aft limit!
If I'd crashed it, I would have had a very hard time explaining why I chose to fly it that way! You take on a lot of responsibility choosing to fly a less than servicable aircraft, make sure it's worth the risk. It's probably better to have the owner fix the plane. He's expected to anyway!
Pilot DAR