Since there hasn't been a post like this in a long time ...
Here, I intend to illuminate what it would be like if the original books included DF canon:
1) Most of the character wouldn't bat an eyelash at supernaturals, or any other strange happenings.
Enjolras: *waves a French flag* Viv-- ... Grantaire, do you realize that you're on fire?
Grantaire: What? ... Oh. Eh, I'm used to it.
Enjolras: And now you're being shot at. *dies*
Grantaire: Well, I've been asphyxiated, bitten, superglued, burninated and what have you. Do you really think bullets are supposed to bother me? I don't think I've ever died, though. Hmm. *dies*
Voldemort: *threatens Harry*
Harry: ... Oh, I've seen scarier, believe me. Ever heard of Zeus?
2) There would be a lot more slash. Well, blatant slash.
3) Misunderstandings would happen.
Evil Villain: And now, I shall send you to your death! *evil laughter*
DF character: He's actually a nice fellow, that Death. Besides, he's not mine. He belongs to Pes-- ... oh.
Lucifer: I wonder how Hitler is faring. I've been slacking on her daily brushings.
Nelchael: My lord? His soul is currently--
Lucifer: ... Nevermind, Nelkers.
4) There would be random lapses into OOC behavior.
Cho: Hey, Quadric! *waters the flowers* :)
Cedric: ...
Cho: ... I mean. Hi, Cedric.
Cedric: ... *goes to his seedy opium den with his 19 groupies. and a pirate.*
Cho: -----------------------------
Christian: Cyrano, help me win over Rox--
Cyrano: *JUMPS ON A TABLE* There once was a man from Nantucket...
Christian: ... *to Roxanne* There once was a man from Nantucket ... ?
Jehan: *waters the barricade* :) :) :)
5) Death (the action, not the personification) would mean absolutely nothing.
Hamlet: The rest is silence. *dies*
Horatio: Now cracks a noble h-- ... Forget that. This is the fifth time he's died today. Fork it up, Claudius.
Claudius: *undies* Aw, man. This is the second time this month you've won the claim to the throne from me. I'll definitely be sleeping on the couch tonight.
Hamlet: *undies* Hey!
6) One word: Miswindows
Beelzebub: The rest is silence. *dies*
The Metatron: Now cracks a-- ... wait, what?
7) "My typist made me do it!" would be the new "It's ineffable!".
>:D
Typist: Not his funniest, but he really wanted to write up a post. :D