Radios that are terrible, inop landing, inop taxi, inop bcn, engines that must be kept low on oil (4qts) as otherwise it spits it out, inop flaps, inop parking brake, inop nav, oil service lid locking mechanism inop and taped down with gaffer tape, etc.
Some very valid points - beacon, flaps, parking brake, oil inspection lid are inexcusable - and some negotiable ones. For day VFR in a light single do the nav lights accomplish anything? Vital for night flight of course, but I can't think of the last time I spotted an aircraft due to a low powered coloured light. I do wish people would always use strobes in day flight though when fitted...
Engines will naturally find their own oil level, since every engine is different. Despite the same manufacturing process every engine out there produces very slightly different power characteristics for example... There is however a delicate balance between not overfilling an engine with oil (eg attempting to keep Lycoming O235 lumps at 6qts all the time is pointless and just leads to a hot oily smell shortly after start, they're much happier kept just above 5qts) and leaving one so low that it's constantly burning it - 4 really should be an absolute minimum for smaller GA engines, not a starting point!