What makes a road 'fun'?

I think a fun road is one that keeps the sense of speed high for the type of road, and allows the driver to "read" the road, rather than rely on trial and error after a million failed laps. Not every corner should be a windowmaker, but every corner should be interesting. Every feature should have a purpose, either geologically or in terms of driving experience.

Cool things that increase the fun of a road:

• Mix of increasing/decreasing radius (use decreasing sparingly!)
• Blind apex, obstructed by building/wall/rock face, tall grass, overhanging branches etc
• Elevation change
• Double apex corners
• Lightly banked chicanes
• Hairpins (sparingly)
• Bump/rut in inside line of corner cause by pothole,bump, pavement edge. To cause a similar effect to hitting the rumble strip on a track.
• Pavement transition to make the car get a little squirelly, but not send it flying.
• Slight embankment on the sides or a ditch, rather than just a direct transition from road to wall of trees. This can be used to allow awesome 3 wheel action, or to help guide the car back onto the road without obliterating it.
• Occasional risky area with no barrier
• Narrow sections such as one way bridges, construction zones, hazards like a rock slide partially cleaned up.
• A corner that "looks" slow, but if taken the right way, can be done more quickly than expected.
• In an open world, alternate routes that trade shaved time for extra danger (ie traffic calming devices, extra hazards, poorly designed/maintained road)
• Nice flow/rhythm in terms of speed, direction and elevation changes. The car should dance around!
• Light jumps
• Interesting consequences. Rather than getting stuck in a tree every time, have the car fly off small cliffs or launch off piles of rocks or junk, into ponds, rivers, back yards of houses, etc.

Infuriating things to be left out:

• Off camber (almost impossible to tell until too late, depends on memory of corner to get right)
• Unrealistic obstacles (most roads do not have boulders littering every shoulder!)
• Large bumps across the whole corner (makes you just have to slow down a lot to avoid launching off corner, no way to pick a better line. Also depends on memory of corner)
• Roads completely bare of rails/barriers (unless they are really really primitive or gravel roads)
• Too many trees too close to the road
• Small rigid objects that snag the car unrealistically (tiny trees on the shoulder, signs, reflectors)

Other Considerations:

• Make the entries to corners memorable with landmarks or differences in the terrain. Otherwise, if the entry looks the same, the corner should behave rather the same.
• Use signage like a real road has to indicate hazards
• Match road types to danger level. Old bumpy narrow potholey roads might have more tricks like off camber decreasing radius corners, but their condition gives away that there is more risk. Whereas a nice road surface should indicate less surprises/better road engineering.
• Most public roads can be travelled at the speed limit without drama. Do laps in the car at a realistic constant speed and see if the road seems consistent in character.

I'm sure I could put down more but maybe this will get your mind turning!