These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Awesome Music: Tons. Many of his songs could be pointed to as "awesome", but the ambient 10-minute-long epic "Strobe" is often cited as his best song by many fans. Other popular choices for "awesome music" include "Ghosts 'n Stuff" (both the original and the vocal mix), "I Remember", "Raise Your Weapon", "Professional Griefers", and "Cthulhu Sleeps", but it's safe to say that nearly any song of his, especially those released in the last few years, could be a viable contender for being awesome.
Broken Base: A minor example. While Flame Wars hardly ever break out, there is nevertheless a distinct rift between the old guard (usually Random Album Title and earlier) and the new fans (usually For Lack of a Better Name and later). This was brought about by his sudden swell in popularity following the release of "Ghosts 'n' Stuff" in 2008, new fans brought in by his newer albums such as 4x4=12 (which generally isn't well-liked among the old guard), and his changing style over time, as seen in "Early Album Weirdness" on the main page.
Fan Dumb: Upon announcing his South Africa tour, some fans were making stereotypical comments about the place and complaining. Needless to say, he wasn't happy about it.
"First off, allow me to apologize on behalf of some (not all) of my ignorant "fans" for their idiotic remarks. Pretty much all of the "fans" making negative remarks about South Africa haven't even left their mommys basement, let alone be privileged enough to visit South Africa."
On a broader note, you have some older fans who cry He Changed It Now It Sucks and It's Popular, Now It Sucks, and stereotype all newer fans as ignorant idiots who only know his "popular" tracks (such as "Sofi Needs a Ladder", "Raise Your Weapon", and "Ghosts 'n' Stuff"). And you have some newer fans who really do only know deadmau5's popular and later songs and cry "this is not deadmau5" when they come across some of his older stuff (shining examples of this can be seen in the comments for "Careless"). And on both sides, you have the people who cry "This Song Is Not House, So It Sucks" every time he decides to bring out something on his SoundCloud that dares to be something different.
Fandom Rivalry: Dubstep fans who don't like Skrillex tend to embrace deadmau5 instead. Ironic in that deadmau5 isn't really a dubstep artist (what with only two dubstep songs actually released as opposed to the many house tracks) and that he was the one who signed on Skrillex's Scary Monsters and Nice Sprites EP onto mau5trap and kickstarted his career.
Doubly ironic in that Skrillex is credited (under his real name Sonny Moore) as one of the writers for Raise Your Weapon.
Friendly Fandoms: His fandom tends to overlap with that of Skrillex's. This is especially evident in how you can spot mau5head-wearing fans at Skrillex concerts.
A lesser example occurs with anyone who's released stuff on mau5trap - Feed Me, SOFI, Foreign Beggars, Noisia and Excision, for instance, have differing degrees of overlap with deadmau5's fandom. SOFI in particular shares many of her fans with deadmau5 because her vocals on "Sofi Needs a Ladder" and "One Trick Pony" started her professional career.
Thanks to the vocal version of "Ghosts 'n Stuff", deadmau5's fandom has quite overlapped with the one of Pendulum.
And in a rare example of crossing genres, his fandom is getting some overlap between his own and that of the Foo Fighters, due to recent collaborations and associations with each other.
Harsher in Hindsight: Any jokes about him loving his cat more than his girlfriend were quickly made this when they broke up.
Ho Yay: Often has it with Skrillex and especially Tommy Lee, the latter of which was taken Up to Eleven when the two of them were blatantly flirting with each other on Twitter.
The above mention with Tommy Lee is summed up pretty well by this image◊.
The Ho Yay between him and Tommy Lee was also lampshaded by Tommy Lee's girlfriend, SOFI, after deadmau5 and Tommy Lee were discussing going to Fiji on Twitter:
"@deadmau5 @mrtommyland now UR planning a romantic getaway? Don't flatter urself Joel - he asked me the same thing earlier today pffft"
Memetic Mutation: "Go to bed Joel" is an injoke in the fandom after it was spammed incessantly on a Q&A session he was having on /mu/ and is usually used as a response against one of his trolling attempts.
"[X]mau5 - [Y] 'n' Stuff", such as "Breadmau5 - Toast 'n' Stuff", which is a parody of "Deadmau5 - Ghosts N Stuff". Variations are often posted all over the place in the comments for mau5-related YouTube videos.
The mau5head itself. Even if you don't know who deadmau5 is, chances are you've seen some form of mau5head somewhere.
Signature Song: Either "Ghosts 'n' Stuff" or "Moar Ghosts 'n' Stuff". The former is used in the vast, vast majority of his cameos and promotions, to the point that fans tend to complain in video comments now about how overused it is, while the latter is a sequel/prequel song that he often mixes with the former at his live shows.
"Professional Griefers" is also starting to get this treatment. deadmau5 refers to himself as a "professional griefer", and the song is easily one of his most anticipated upcoming releases and set performances.