Every BSO musician has his/her own ideas about what makes a good conductor. Here's what I think is needed, both personally and for the BSO, having been a part of so many concerts over the years:
Anyone auditioning the other day had to 1) show she/he understands the music being conducted, and 2) show that she/he can communicate that understanding to us BSO musicians. Neither of those criteria is as easy as it sounds. And in the case of the BSO, I have noticed that we respond well in general to rather extroverted conductors, the ones who can really "put it out there" for us to see and respond to, so to speak.
When a conductor "understands" the music, they know which tempos actually work, both from technical and from musical perspectives. They must show (sometimes in anticipation) changes of dynamics (loud and soft, decreasing and increasing) and changes of tempo very clearly so that all musicians execute those changes in the same way. They understand when their job is to be "traffic cop" and keep us together, and when to go slightly off the beaten musical track to provide an interesting detail we hadn't considered before.
Communicating the music might be the toughest part. I am amazed sometimes at how much conductor's facial expressions influence how we perform music, and how we sense that conductors are communicating with us. It is important to show that you are involved with the music, that you don't meet every different section of the music with the same poker face, smile, or grimace. On the other hand, too much emoting and even smiling can be off-putting. Dirty looks, even for an obvious mistake, are an absolute no-no.
I also like conductors who use their whole bodies to conduct, as opposed to standing very still and mostly using their arms. When you are a musician toward the back of the stage, it helps so much to see the conductor's movements peripherally. You frequently can't take your eyes of the printed music, or even your instrument at times, to look directly at the conductor, so her/his movements need to be big enough to see, and of course, they have to convey the appropriate musical signals.
I'm curious. As an audience, what do you look for in a conductor?