Sometimes, its the small things in a game that make you happy. Just list a few things like this. I'd probably say, when hitting CTRL+F, the FPS stays there even when the menu is disabled. That's smart.
When you have the Christmas mod, when you honk the horn of a Winter mod vehicle, the horn plays the tune of Jingle Bells.
I like the Pigeon's little yellow piece of paper that says "WARNING: DO NOT MAKE A STEERING SHARPLY AT THE HIGH SPEED". That's a really cute little detail that makes it so much funnier to drive. Also, the after-fire effects, engine rocking and chassis twist, and the LOL bottle that were added in 0.11 are really quite nice. Finally, I think it'd have to be the sheer level of detail included in WCA. It's so awesome that I can almost forgive its incredibly-low FPS on my machine (15 FPS is the max I've gotten on it without hiding the city). Almost.
Multiple maps. Without them, the game could not have nearly as much location diversity as it already does.
the detailed mechanics(suspension) how unforgiving it is if you can't drive but that's the core game... little details are: realistic-ish simulation of crazy machines (flying pigeon and maluch) being able to drive anything anywhere any way... (loaded truck race on the dirt...) but the best part of the game for me is the parts selector... being able to set up a vehicle any crazy way you want to funny details: how cars don't all have water reservoir, battery, brake fluid reservoir and auxiliar, power steering reservoir, windshield water spray reservoir, eletrical wiring and etc... and also how commands still affect brakes, steering, engine and what else you have no matter how badly everything has been torn apart... how time speed doesn't affects foliage...
The idle sound on the V8 Vanster, Grand Marshal and Roamer. (It's so realistic) And the ability to start/stop engines