[Sirref]

This depends on the type of rider and type of riding

when I am racing:
I am impressed by those at the front of the pack that I aspire to beat. People like O'Hare and Paasch who impress me for different reasons.
O'Hare has a indestructible confidence on two wheels that no matter what he does on the bike he can control it leading him to charge into corners in ways I can't imagine while still holding pace. I admire his ability to ride anywhere on the track and maintain pace (he's passed me on the inside of T3 NJMP with the back clean out like he was riding a dirt bike and I still have no understanding how he stayed on the track to make the chicane)
Paasch impresses me with his ability to be incredibly calm and collected at extreme pace, his bike never looks out of sorts and when he's behind you the pressure is insane. He shows a wheel in almost every corner waiting for you to make a mistake and let him through. It feels as though you are being hunted, in fear that every corner he's going to try to slip through. No one else has the ability to disrupt my thought processes from behind like he can.

When I'm coaching:
smooth inputs, good eager attitude, clean passing, and the ability to immediately take knowledge and put it into practice is how you impress me.

I recently got into flat track racing so I'll post this one up too:
There are three things that impress me with other flat track racers. One is the ability to stay up when bumped at the apex, the other is the ability to smoothly control the rear brake. The third is the completely insane mentality I see in many flat trackers, sort of a "**** it" mentality where they'll crash 20-30 times in practice to find an extra tenth on the slick surfaces and get back up immediately to do it again in the next corner.