I have to wonder what these peoples' definition of a good actor is. Not a great actor, but a good one. And we're talking television here (mostly), so just leave out any comparisons to your favorite Shakespearean stage actor (not that I don't think Shatner could be a stage actor-- I think he'd be great on stage, actually).
Here's my definition: when someone comes on screen and has such a presence that you can't take your eyes off them, they're a good actor. I was watching Boston Legal the other night, and even at 75, the man steals every scene he's in. And he does it on a show where he's surrounded by other good actors (Candice Bergen, James Spader).
Every single line is a performance for this guy. I don't think he's ever delivered a boring line. When he starts talking, you want to know how he'll finish. He oozes charisma. This is a guy who almost single-handedly created the Star Trek phenomenon just by oozing so much damn charisma. (And playing off of Nemoy, of course, whose purposeful lack of charisma only amplified Shatner's performances). Have you ever seen that Trek episode that shows the original captain? How long do you think the series would have lasted if that guy had stayed captain? Think people would still be going to Trek conventions today?
People call him a ham and say he overacts... both of which are true. But he overacts in an interesting way. Soap opera actors overact in a bad way, like a teenager throwing a fit because he can't take dad's car out. Shatner overacts like a circus barker. He sells you on the idea that you want to hear whatever he says next.
Most importantly, he's self aware in a way most actors aren't. Think of all of the crap acting that litters entertainment. Do you think those actors know that they're horrible? No, because they're not aware of the audience. (This is why comedians often make such good actors-- they're aware of the audience because in stand up comedy, a bad performace gets you booed off stage). When Shatner over-delivers a line, he knows it. He does it on purpose.
And that's a good actor in my book. Not a great actor, but a good one. He makes you want to watch him, and that makes him better than 99.9% of all the performers I've ever seen.
I should point out that I'm not even a Trek fan. I used to watch the original when they would show reruns on Sci-Fi, but that's it. The Next Generation always annoyed me-- pretty much all "spaceship" sci-fi annoys me (except for Firefly/Serenity, which, to me, is some of the only good "spaceship" sci-fi ever produced). None of those shows have any spark, any magic, any charisma, and I still can't wrap my head around the fact that no matter what crap they show, it'll still get ratings if it's on a spaceship. Someone wanna unwind that ball of yarn for me? Because I don't get it.
I am a Boston Legal fan, now, because Shatner made me one (and Bergen, and Spader). Despite the sometimes annoying camerawork, they make me want to watch the show, even though I generally hate courtroom dramas (and police dramas, and FBI dramas, and crime lab dramas, and government dramas... they make me want to boil my TV, and I don't even have cable).
So lay off Shatner, people. He may be no Pacino, but he's not trying to be. He's 75 and still stealing scenes. He's worth watching, and that makes him better than almost every other TV actor out there. My TV only gets two channels, but he makes me happy that his is one of them.