When I did mine; it was doing a couple of odd things.
When rolling to a stop or idling along in traffic; it would idle too high, and fight you on the brake until stationary. Then the idle would drop, but not to 650rpm.
When starting; it would do a funny little hiccup, as if the fuel pump hadn't primed properly; then the idle would settle; again not at 650rpm, normally a bit higher.
When I opened it up; I was quite disappointed. I expected it to be absolutely filthy, as most other people had discovered. Still, I flushed it clean, and I too lost the painted on coating at the sides of the throttle plate.
It obviously worked though; when I refitted it, I had no more histrionics or issues, and it definitely idled higher. A quick Idle re-learn; bang on 650rpm, everytime.
The coating is this stuff apparently; http://www.dowcorning.com/applications/search/default.aspx?R=740EN