It's very hard to notice but, in Mr.Doppleganger's Dialogue portrait we can see another face inside his gaping mouth. The only time you can see this on the character model is after you defeat him. Though the text boxes may obscure it.
Kinda, When Travis and Badman teleport into a game world, they are naked (as a reference to The Terminator), however their genitals are not visible. (Though according to the ESRB their butts can be seen, which is why the description says "Partial Nudity")
You save the game by using the toilet. Complete with an animation of Travis or Badman sitting on it, with a text box that says "NOW SAVING" coving the crotch. Worth noting is that if you are playing as Shinobu or Bad Girl, you don't see them using the toilet instead a pixelated screen with the "NOW SAVING" text shows up.
In the bubblegum fatale DLC you unlock a complete version of Killer Marathon, one section of this world is set in space where you need to use air bubbles to recover oxygen.
Kinda, Silver Face's boss theme has lyrics, a certain word is muted but words like "fuck" and "shit" are not. It's pretty obvious that the word being muted is the n word. Also worth noting that it isn't being used in a racist context.
SPOILERS: After the credits, you enter a test room with a third person camera and a test dummy, Travis then tells the player to leave before teasing No More Heroes 3. (It's worth noting that skipping the credits will not skip the after credits sequence.)
SPOILERS: Not until the final level CIA. Which is not a game but set in the CIA, Both Travis and the player see the agents as Bugs as a result of the Death Drive messing with his vision.
This being a No More Heroes game, the fourth wall is sometimes broken. This probably doesn't count but it's worth noting that the game is a metaphor for Suda51's life as a game dev.
There are drug references, such as a weed store in the first level (hard to notice). SPOILERS: The "coffee" seen on mars might be some kind of drug like Marijuana but it isn't clear.