Hey, It's That Voice!: Almost by definition. Don't fret if it takes you a while to hear Kevin Murphy as himself and not picture a red gumball-machine shaped robot.
Admittedly, many fans try to pretend Kevin is a red gumball-machined robot and Bill is a golden beaked robot.
It mostly works fine that way, except that it can be disorienting when one of them references something in real life like his family, and you have to remind yourself that they aren't robots on a spaceship anymore.
One of Us: The commentaries feature frequent references to nerdy or geeky activities/interests, unsurprising considering we're their target audience. In the Harry Potter Rifftrax, several jokes were made referencing things that were either only in the books, or hadn't been introduced yet in the films (like a joke involving Horcruxes before the Half-Blood Prince film had been released). In the Lord of the Rings rifftrax, the guys gushed about Middle Earth and its rich history, like Mike giving a detailed explanation of the meaning behind Aragorn's war cry "Elendil!" Video game references are the most frequent and generally occur with the least provocation. Mike makes jokes several times that hint he spent most of his teen years playing NES games, and jokes about more modern video games like Portal get brought up as well, though they have more of a We're Still Relevant, Dammit feel to them sometimes.
It seems that Bill Corbett is a dyed-in-the-wool Whovian — during the riff of Doctor Who And The Daleks, he was the one who made riffs about the movie not being canonical, and during a Christmas live-riff show was seen wearing the Fourth Doctor's scarf.
Throw It In: If they think a joke is particularly funny the crew will leave their laughter in as opposed to editing it out. Sometimes it can go on for several minutes and almost derail the commentary.