Ah, you've touched a nerve here as I have often stated in these hallowed (web) pages that not only is Joni Mitchell (one of) my all-time favourite songwriter(s) but Shadows & Light is probably my all-time favourite live concert. (Get yourself the extended DVD version if you can). What really makes this gig one of the best (imho) is the excellent Pat Metheny (all-time fave Jazz guitarist) and Jaco ('nuff said!). You also mentioned Mike Brecker (brilliant) but I'll just give a huge shout out to the incredible Lyle Mays on keys (Pat's long-time musical partner) and finally highly-respected drummer and percussionist, Don Alias.
As for Joni Mitchell's guitar work being 'pretty good' - obviously not a lead guitar player, as such, she actually 'invented' many alternate tunings for herself. In short, this was because she contracted polio as a child and found it difficult for her left hand to play regular chords in standard tuning. (There's a ton of stuff online about JM but jonimitchell.com is a good place to start).