Being a broadcaster has got to be really, really hard, especially for baseball games. They have to sit in one spot for three-plus hours and think of things to say that are funny, fresh, informative, and digestible, usually on the fly. They have to do this 162 days straight, sometimes for crappy, uninteresting teams. It's a plum job, but let's not pretend that any average Joe can do it.
That disclaimer out of the way, broadcasters vary widely in quality, and it feels like there's more poor commentators than good ones. For example, see the Joe Buck/Tim McCarver duo that headlines all Fox broadcasts (they're doing the ALCS and the World Series). What, exactly, are these guys bringing to the table? Buck sounds like an assistant principal addressing a school assembly, not a guy narrating a thrilling playoff game. And McCarver routinely flubs lines and makes inexplicable, unsupportable statements (a couple days ago, he said Victor Martinez is a top three hitter in the AL. Martinez is 36th in the AL with a 112 wRC+). McCarver is like Yogi Berra, but without the sense of implied wisdom or bonhomie.
But it's easy to criticize. It's more constructive to figure out what makes a good broadcaster.
I'd say Vin Scully is nearly a consensus pick as a top broadcaster. He has a pleasing, mellifluous voice, excellent baseball instincts, and a vast repository of stories to fill any dead air. Scully can make any game watchable by pure charisma. And he just sounds dang likable.
So I'd like to enumerate some criteria for what makes a good broadcaster. Your mileage may vary concerning which criterion is most important, but I think most people can agree that these are all important qualities for a commentator.
1. Excellent, precise command of the English language
This sounds obvious, but man, it's abused so, so often. McCarver is a serial butcher of proper grammar and syntax. I don't particularly want to cast stones at the Rockies booth, but George Frazier and Jeff Huson (particularly the former) tend to take the shotgun approach to speaking: just spew everything out there, as fast as possible, whether it hits the target or not. They never use four words when forty will do. Drew Goodman, though, I think is excellent in this category.
The key word here is "precise". Every play in baseball is unique, and describing it properly is the main job of a broadcaster. So it bothers me when guys in the booth rely on stock phrases or terminology that don't actually apply (Huson, in particular, seems to only have about six default explanations for anything that happens on the field).
In my personal opinion, this is the most important quality of a broadcaster. He could have Pee-Wee Herman's voice and Joe Morgan's opinion of statistics, but if he can accurately describe what's happening in the game, he's most of the way there to being a solid broadcaster.
2. Baseball knowledge/instincts
This one should be pretty obvious. Do they know what they're seeing? Can they differentiate between a slider and a curve ball? Do they know when a pitcher has a good move, or which outfielder has a great arm, or know when a pitcher got lucky with a hanging breaking ball? These details bring texture to a game, and they can elevate a good broadcaster to a great one.
3. Statistical knowledge
We all know that baseball is a game of numbers, and a broadcaster has to know how to use them (although I'm sure most of the stats they bring up are fed to them by other people). At the very least they should know about batting average, on base percentage, slugging percentage, and ERA. Ideally they would have a working knowledge of more advanced concepts; batting average on balls in play, fielding independent pitching, defensive metrics, park effects, Wins Above Replacement, etc.
Almost as important though, they should know when not to use stats. I always roll my eyes when broadcasters remark that a hitter is two for five in his history against a certain pitcher. He's hitting .400 against him! So the batter obviously sees the pitcher really well and is going to get a hit!
If broadcasters were more aware of sample size issues, regression to the mean, and randomness, a lot of meaningless statistics wouldn't be bandied about like they were important.
4. A good voice
Hey, not everyone can be Vin Scully, but having a interesting, pleasing voice is pretty important if we're going to be listening to you for three hours straight.
I won't want to hang out with you if you sound like a jerk or a bully. Among Hawk Harrelson's many issues as a broadcaster, he's such a relentless, shameless homer that he sounds like kind of a tool.
6. Chemistry with partner
For some reason the typical broadcast booth has become a two or three person show; one person is the "voice guy" and the other is the "color guy", who is usually a former player. When these duos are firing on all cylinders, the back and forth feels easy, natural, and pleasant. If they aren't on the same page, there can be awkward dead air.
Anyway, I'm sure there are other important aspects of being a broadcaster, but this is starting to meander. It's a hard job, I'm sure, but it would really be nice if people got better at it.