How off-the-cuff they're able to be, and not sound rehearsed. JTo paraphrase something from TF, 'Joels' work far better in that medium than 'Mikes'.

I like the kinds that use a bad game, really. It's so easy to lampoon, and have a great time watching. Conversely, I hate any that are about overworld exploration, 'cuz I always find myself going "NO GO THIS WAY URRGGH HOW THE FUCK DID YOU MISS THAT"

I dislike it when the LP has no editing what so ever, I don't want to watch you lost in a cave for 4-6min or watch you grind.

I like LPs that have some kind of gimmick, like a catchphrase or something.

I like Let's Plays that can take advantage of an absurd situation. Take something funny that happens in the game and just magnify it. Now I know you said you don't like Let's Players who use a camera to record, but Darksydephil is a great example of someone that improvises these gaming moments. Just go with the moment, react how you would normally, just "out loud," and you should be alright on this.

Now for things I hate. Talking over in-game dialogue way too much. DSP is certainly guilty of this :P Another thing is not spending much time on exploration in games that allow it, such as Red Dead Redemption, as an example.

That the person narrating them has a "personality", something entertaining and funny that keeps you interested. Usually, more than one person does the trick, too.

Gotta have some sense of humor and character. I like a little bit of hype, too (which can be hard to do if the game is too easy). Also its better if the person playing knows what they're doing.