A week or two ago, I bookmarked a thread on /m/ in there was a shitstorm brewing about whether or not KILL la KILL discussion belonged there. The in a few days, the thead 404'd.
My first thought was not “Shit, thread’s gone,” but “Oh, hey, look, Yotsuba!” It was then that I realized the brilliance of using her for the 404 page: having a cute loli, and what’s more, a recognizable cute loli, pop up with every 404 message eases the frustration of getting a 404 message. It brings a smile to my face every time.
I love 404-chan, and you should, too. I don't even read the manga! I'd make her my waifu, but... you know... she's a loli... that might seem creepy...