Keldor, I think, just doesn't like Kate Mulgrew's voice regardless. I found her grating at first myself. My point, though, is that she sounds consistent with Voyager (even if you hated her in that), something that is hard to do.
With Patrick Stewart, I think his being a stage man is what works against him. Picard's voice was a huge, loud, dramatic stage voice, and he (Stewart) doesn't attain those kinds of energy levels reading into a microphone. It's understandable. He also tends to get a lot of the most boring stuff to read for games. Like in BC as a tutor. Thought he was good in that despite it. Hidden Evil he was good. Just not the Picard level intensity we're used to from TV and film.
I think Shatner has the same problem as Stewart. No audience, no energy to work with.
I'd love to see either DS9 or Voyager come out of left field with a PC title. Of the 5, I think those 2 Captains are best suited to voice over work. Makes sense, they both have deep, characteristic voices, they probably have both done voiceover work prior to Trek I would guess.
But no thread on voiceover is complete without mentioning Dorn, who is great at it.