Very recently (as in, not even 30 minutes ago as of this writing), I was at a jam session in Chicago. It doesn't necessarily matter where, and in fact naming the venue and the people might distract from the point I have here, so I'm gonna try to make it nebulous.
For whatever reason, people felt like I was playing well. Lots of 'you sound great bro' from throughout the room, things of that nature. If you've played at a jam session full of professionals, you know how that is and how it feels--admittedly good but in a pretty insubstantial way, like a big boll of cotton candy or a silk pair of underwear.
There was one small comment which went against the grain. After I held in there (more or less) against a tune the leader of the jam session called, in a split-second spare moment he said to me in an offhand, quiet, but directly worded way: "You need to learn how to deal with the changes more".
Here's the thing: the guy was right.
I know, at least subconsciously, that I've been slipping in terms of really exploiting and engaging with basic V7-I relationships in a consistently creative way. I have the skillset to avoid that weakness (through a small library of substitutions and superimpositions I've been accumulating over the past decade-ish), but my strictly diatonic, functional-harmony material lags behind.
Out of everyone at that jam session, the leader was the only one to see through that particular avoidance mechanism (along with any technical pyrotechnics that I may be able to do) to the problem behind it, and then let me know the problem was something that deserved attention. Furthermore, he let me know in a personal and quiet way that would ensure I heard it, as opposed to just listening to it, and in a direct way, so I couldn't mistake cause and effect between behavior and commentary.
That's unique and more than worth the price of admission (of course, the session was free, but I digress).
I think that people who are capable of doing that, both in terms of perceiving the problem and in terms of letting someone know immediately and gently without ego, are few and far between. I think it'd be silly to not only start really dealing with the vertical structures of the tune I'm playing more, but to start considering how to express my criticisms in that way.
That is, to make them wanna put in the effort to make themselves better. I think I'm gonna revisit Warne Marsh for a while. And learn more tunes. But I could always stand to do that.