Skid marks and damage obviously. Livery editor isn't essential to me.
In fact, I hate the idea of the livery editor. 95% of cars with liveries look like crap especially compared to originals.
Not sure exactly where you are looking, but for the most part there are some great looking designs out there. Yes there are some eyesores out there, but not near as many compared to some of the stellar looking designs. Creativity is a great thing, and adds a WHOLE different aspect to the racing genre. With a livery editor you can do plain and simple designs, it doesnt always have to be neon colors with anime characters painted on them. Just adding a simple stripe, two toned colors, or race decals can get the job done without overdoing it. It sure beats the hell out of online races that have the same 4 basic red Ferraris or the same exact yellow Ford GT's with black stripes. A livery would give you a personal bond to your car, making it feel like it is truely yours and special. But I guess if you don't mind seeing 4 of the same exact cars looking identical then thats your preference. A livery would just spice online up a bit, leaving the dull same single color cars as a thing of the past.
And what happens when your favorite specific model of car is left out? Hit the livery and add the colors/patterns/decals to transform that car into its counterpart.
Example: Maybe the standard '96 vette is in the game but no 'Grand Sport' edition.
Turn this:
Into this:
...tweak the performance and boom...you got yourself a whole new model of car that wasn't included before. That could go for any specific car model that wasn't in included in the game. As long as you have the base car model, you can have any special edition/racing version you like. Another example: In Midnight Club LA, it had the 2008 Corvette ZO6. I took it to the editor, slapped on some simple "ZR-1" decals I made, upgraded the performance, and whalla...a ZR1 was born (not 100% accurate but you get the idea). I've seen alot of people doing this because they had the base model but not the upgraded or special edition. A livery doesn't always have to mean neon green cars with pink teletubbies on the side.
EDIT: By the way, I voted for damage/tire marks because that is a must have for a racing sim. My post was just in response of him saying he hates the idea of a livery editor, and 95% of them look like crap. Honestly, there's no reason not to have both. Its almost year 2010, neither one is a stretch to have, and both would be a huge addition to the GT series.