Kesling is not a bad announcer because he followed John Ward and Ward was so good it makes Kesling look bad. Kesling isn't all that good because he simply isn't all that good.
He is not very fluent or smooth in his delivery. He makes a lot of mistakes in pronunciation and trips over words (e.g, there was a "Garrett Jarantano" during Saturday's game). There are huge "gaps" in his descriptions of plays. For example, a 35-yard touchdown run with a couple broken tackles or making guys miss goes something like "Ty Chandler gets the carry...he's got running room...to the 5...to the checkerboards!" You're sitting there listening and when he says "to the 5" you're thinking whoa...in that time he had run all the way from the 35 to the 5?
The punt block in Saturday's game went something like "ETSU to punt...blocked...touchdown!" As a listener you have no idea that it is shaping up to be a big play. You hear the crowd get louder, then Bob simply declares it was a TD or a big gain a second or two after it is over. His phrasing and things he says are just awkward sometimes (remember the "what a turn of events" line after the Hail Mary against Georgia?). He just isn't very descriptive in his calls, and good radio announcers give enough detail for you to imagine what that play might have looked like in your head.
I think Priest does OK...I'd like to hear a little more analysis about what occurred rather than pointing out the obvious, but he does bring a lot of energy and you can tell he loves the Vols.