O-10, "Idle Moments" has been on my list of "to buy" for literally years ever since a friend played that cut for me. It was of particular interest because it was, as you said, one of Joe Henderson's earliest things on record. Thanks for the reminder. That recording is absolutely incredible with a beautifully relaxed feeling. As you said, all the solos are great, but Joe.....wow! There is a saying among jazz players that "you can't play outside until you know how to play inside". What they mean by that is that a player has to master improvising "inside" the tune's harmony in a traditional (easily recognized by the listener) way before he can leave that harmony for more "adventurous" note choices "outside" those traditional harmonic constraints. This has been an area of considerable controversy. Many so-called "free" or "avant-garde" players have been shown to be nothing more than imposters since "musical political correctness" has been reluctant to call their playing what it is: bullshit. Joe Henderson is a master at leaving the traditional harmony in a beautiful, organized and CONTROLLED way as a way of EXTENDING the harmony, and not simply destroying it in the spirit of being "free". Two moments in his solo that demonstrate this harmonic mastery are 8:35 and 9:45; it's a kind of harmonic spiciness.
If all that wasn't "geeky" enough: Joe Henderson plays with a very distinctive and uniquely warm sound. Part of the reason for this is his choice of mouthpiece. One will notice that tenor saxophone players play either a metal or rubber (plastic) mouthpiece. During the "swing" and early "bop" eras most players played rubber mouthpieces. As players developed more aggressive styles in the "hard-bop" eras and beyond, they sought the louder and brighter sounds that a metal mouthpiece gives the player. Joe Henderson is a notable exception to this trend. Not only did he choose to play a rubber mouthpiece, but it is a mouthpiece normally used by classical saxophonists who require an even more rounded and less edgy sound.
Another interesting thing about this recording is how the tempo relaxes even more from beginning to end. Sometimes this is intentional; it usually isn't and is just something that happens in the performance. In this case things start to relax a lot during Hutcherson's solo. Notice how much slower the tempo is by the time the melody returns at the end (13:12); it's not a subtle difference.