From what I've seen in previous models I've build the most important thing is the body paint and the lines of the body and that's because it's gonna be the first thing it pops up in your eyes when you see the model.
I've seen models with a really poor paint job. Dots of dust in the paint, little scratches, etc
Also the lines of the body are a really frustrating thing when they don't come together and you have big gaps between body parts.
Another thing are plastic parts.
It makes a huge difference to have good quality plastic parts. I can't be specific with this since I don't know what makes the difference, but you can see the difference with the naked eye, or you can feel the texture between plastic parts used for the Lamborghini Countach and Mercedes Gullwing. For Lamborghini, all plastic parts have a nice texture, don't have scratches or unfinished surfaces and the plastic is solid and doesn't give you a sense of cheapness.
I hope the tires are gonna be rubber and not plastic (rubber). Again i have the Lamborghini example. It's the only model I've build which has nice tires. All the other models have tires made of some sort hard rubber wich feels like it's gonna break if I drop it.