There is one thing that holds true for any type of music: good time is critical. A musician should be able to nail the beat right on the head! Even if a piece of music is free or rubato, great musicians know exactly what they are doing in terms of the timing of their playing.
Your relationship with a metronome is important- you can either ‘follow’ it, play right down the middle of the beat, or ‘lead’ it as if you are showing the metronome where the beat is rather than vice versa. In my opinion, following the beat can make a rhythm section player sound timid, while playing down the middle or leading the beat slightly sounds confident.
My favorite jazz bass players will either play down the middle of the beat, or lead the beat slightly. This contributes to the rhythm section sounding bouyant, energetic, and confident. It also sets up a great background for a soloist to play slightly behind the beat with confidence. I feel like Ron Carter puts it down the middle, while someone like Ray Brown or Dave Holland may tend to lead the beat a little.
Timing for a jazz solo is absolutely crucial. Listening to a recording of my trio gig last Sunday, I noticed that on many organ solos started out with washy, indefinite time, and then grew in confidence as they went along. It really bugged me because I like a solo that starts simply and confidently. I don’t really even care about wrong notes if the time is killing.
I guess that’s what I’ll be working on for the next few months.