What I like about good jamming is the exploration of possibilities, and what makes it good is the level of imagination, or perhaps how well that imagination is realized. I understand this can be very subjective. Personally, I prefer jams that remain a bit grounded, at least to a point where the source material is still recognizable, or at least relevant, even while the music is taking you places. That is, you should be able to tell what song is being played, and each one should be distinctive - each song has its own characteristics, and shouldn't be just an opportunity to cycle through each band member's collection of riffs each time. Another show on the above list is a club gig by War from the late 90s. Over an hour into the show I realized they had done only four songs. You really don't need to give every guy a chance to stretch out on each song.
For me, a good performance incorporates both approaches in good measure, and I try to incorporate this into my own playing. I like it when a jam, or even just a solo, is worked so that it returns to the song and resolves to the melody. I keep this in mind at shows, and try to reel it in within some sort of set time frame. Sometimes the singer lets me run a little wild, especially if the spirit is upon me; sometimes I bring it back a bit sooner, if that's how I feel it. I also derive a lot of satisfaction from how well I play the little riffs in the middle of a song, or create ambient textures that fill out songs and give them distinct characters. These may be much shorter than extended solos but can offer as much opportunity for creative expression. In my other band, in which I am playing all original blues and rock, I have to keep some sense of how far to stretch out and when to turn the instrumental section back towards home. However it goes, just as in gymnastics, it's important to stick the landing.
Like Casey Kasem used to say, keep your feet on the ground, but keep reaching for the stars.