Well above all else it has to be entertaining, generally this means adding in lots of satire and humor over the game being played. Whether or not the creator is any good at coming up with satire or humor is the real question. Take Spoony, for instance. His wit and humor just comes naturally, and as such his playthrough of Phantasmagoria II is frickin' hilarious and a pleasure to watch. So are his playthroughs of SWAT 4.
Like all things in comedy, you've either got it or you don't, it's that simple. Some people can make great jokes on the fly other people can't. The guys over at Rooster Teeth make good Lets Plays because they're all just a bunch of goofy bastards that play off of one another. Other lesser known Lets Players just seem like they're trying to force their jokes.
But it's not all about comedy, as I said, it's more about being "entertaining" in general and that doesn't necessarily mean being funny. It's just that going the comedy route is the easiest way to get to "entertaining" so long as you're good at it. The over-all point is that a Lets Play has to be as entertaining or more entertaining as if the person watching it were just playing the game...because if it's not as entertaining or more entertaining than playing the game yourself, what's the point of watching it?
Like all things in comedy, you've either got it or you don't, it's that simple. Some people can make great jokes on the fly other people can't. The guys over at Rooster Teeth make good Lets Plays because they're all just a bunch of goofy bastards that play off of one another. Other lesser known Lets Players just seem like they're trying to force their jokes.
But it's not all about comedy, as I said, it's more about being "entertaining" in general and that doesn't necessarily mean being funny. It's just that going the comedy route is the easiest way to get to "entertaining" so long as you're good at it. The over-all point is that a Lets Play has to be as entertaining or more entertaining as if the person watching it were just playing the game...because if it's not as entertaining or more entertaining than playing the game yourself, what's the point of watching it?