I agree with the initial poster, however I was surprised that when I listened to Donny Marshall on CelticsStuffLive he was intelligent, made some interesting observations, and that I actually learned something from listening to him. My main complaint about Donny Marshal is that when he is broadcasting games or in the studio at Comcast I never learn anything from his commentary: he either says something obvious or nonsensical. I assumed that was because he is not very smart, but after listening to CelticsStuffLive, I realize he is very smart and knowledgable.
I am not sure why that does not show up on the air, but my guess is that he is not yet comfortable, does not know what to say, and just tries to fill time. Hell, I can do great commentary when watching the games with my friends, but if I had to do it on live TV then I'd probably sound like a complete idiot. I have also noticed a reluctance on his part to say anything remotely critical about many players and coaches, as many are his friends: I feel like he holds back at times (which Tommy never does).
In any case, he can be a complete genius, but if he cannot communicate while broadcasting (and he cannot), then he is a bad broadcaster.
I remember in the early/mid 1980s that I did not like Heinsohn as a broadcaster. But then I realized it was only the nationally televised games that I disliked him on. (In the early/mid 1980s Heinsohn did the CBS broadcasts with Dick Stockon in addition to the local broadcasts.) Heinsohn would really dumb down his commentary during the nationally televised games. I assume the network told him to do this (or maybe he tried to keep it at a level that Dick Stockton understood). My favorite broadcast team was Heinsohn and Cousy as I learned a lot from listening to Cousy (and he still followed the league fairly close back then). I feel like Gorman has significantly improved since the 1980s (when he was very deferential to Tommy and did not add much IMO) and that they have become an outstanding broadcasting team (if you are a Cs fan that is). When either Tommy or Mike decide to retire or are unable to continue (hopefully not for many many more years), then I hope they bring in as a new TV team Max and Grande from the radio.