I've got a million thoughts I could share in this regard, but I'll confine myself to a few:
--Knowledge married with
joie de vivre goes an awfully long way for me. Romo sounds like a giddy schoolboy at times, and Collinsworth's folksy "I don't know, Al..."-type delivery has been lampooned by Bill Simmons to great effect, but both guys are able to convince me each and every time out that they love the game and know the game inside out. (I voted Nantz/Romo and Michaels/Collinsworth in the poll.) In contrast, there are a few guys like Thom Brennaman out there who seem to aggressively dislike many of the games they're commentating on - Mark Lawrenson, the BBC soccer analyst, is another who springs readily to mind in this regard. It absolutely crushes my soul to hear that sort of attitude on the air.
--Speaking as a commentator, the ability of fans - SoSHers included - to hold grudges against commentators frightens the hell out of me. I think it was Collinsworth who when being interviewed by Simmons said that fans of all 32 teams think he's biased against their team and/or biased toward their least favorite teams. I mean, I think Collinsworth's "I looked him in the eye..." stuff from the Super Bowl was a mistake, but some of you guys would derive so much more enjoyment from his work if you could forgive him for it.
--Cognitive skills diminish over time. To address
@chilidawg's above question, I never really rated Dick Stockton in the pantheon of great commentators myself - e.g., his call of Fisk's Game 6 homer in 1975 is one of the all-time worst calls of a great sporting moment (particularly the way he says "home run") - but it's clear to me that he's way below his peak. I think Michaels has slowed ever-so-slightly himself, but far less so than Stockton.
--Speaking of Michaels, it amazes me how much he controls his voice during games: no good commentator says "touchdown" less excitably and in a lower-pitched tone relative to the rest of the words which came before it as often as he does. (Pat Summerall maybe did, back in the day; Joe Buck certainly did when he was younger, but not in a good way.) And yet, he does this without sounding any less enthusiastic than his peers, and without any condescension. I don't quite know how he does it, although it helps that he's got a great set of pipes. You compare Michaels against guys like Tessitore and Gus Johnson, who make getting a first down on 3rd and 6 seem analogous to landing on the moon...well, I don't even need to end that sentence, do I?
--Joe Buck has gotten a *lot* better down the years. I think his voice is much harsher on the ears than Nantz's and Michaels', but I think he's now getting the most out of it, and his sense of humor can be really underrated at times.
--"Gumbel/Green" in the poll should read "Gumbel/Green/Arians". (Greg Gumbel hasn't exactly been blessed with the best partners down the years, has he?)
--I think about Randy Witten as a commentator the way I think about young quarterbacks operating in Jeff Fisher offenses: we'll never really know how good he is until he's got a new coaching staff and new talent around him. I wonder how he would have fared alongside Sean McDonough, who I rate very highly in all areas except at the very end of games when making the calls that people remember the most.