Good sportscasting is really simple:
Do not detract from the event but rather add voice and insight to the game.
That's it. And of course always remembering we are there for the game. Being authentic is key, as people, even children, are especially good at spotting a fake or someone pretending -- just think of Dick Vitale the last few years as compared to the Dicky V of the late 80's.
A steady, low-key voice for commentary that can be punctuated for emphasis on words like fumble and touchdown, not words like hand-off, pass or even blitz. Shrill is districting and annoying; punctuated low-key is not. Crude, crass or negative have no place in the booth as children watch the games and who really wants to hear a nasty view or opinion while we are having fun. Keith Jackson set the standard and announcers like John Saunders carry on Keith's example.
Have fun with it, as we viewers want to have fun while watching the game, no reason the sportscaster can't enjoy it too; authentic enjoyment is infectious and the viewers catch the vibe of the sportscaster. We all want to have a good time watching the game, do not get in the way of that. Sport is exciting because on any given play, he-could-go-all-the-way applies.
Pre-game, post-game and half-time shows are a different beast then calling a game and all bets are off as that is where the sportscaster is the star(there is no game to watch.) College Football Game Day is a great example where we viewers want watch the sportscasters to learn what they have to say about the day and to enjoy their antics and camaraderie as we share the love of the game.