I agree with IO540 and mad_jock. In my limited FI experience the two principal observations I have are (i) less than full appreciation of aeroplane performance, both as a concept and with reference to what your individual type can achieve; (ii) poor technique. This latter point applies to both students and qualified pilots. I see people nail the nosewheel to the deck and use other techniques at variance with the published technique in the FM/POH. But what really gets my goat (even with my own students, because I think I do a better job than this) is the acceptance of up to a balls-width of sideslip in the full climb. Boy does that erode rate of climb, and putting the ball in the middle soon points out how much more one'll get performance-wise.
As an aeronautical engineer I can't see the sexy modern types, being of the same basic configuration as the older types, having significantly better glide performance than the older ones. 10:1 is as good as you're going to get and even if you could get 12:1 it'd improve your range by only 20%, or about 1/3 of a nm from typical circuit heights. Again, poor technique would soon erode that. On the other hand maybe 1/3 of a nm might be the difference between a crash and a successful forced landing....