Classic image of Hell: fire and brimstone.
Classic image of a dragon: great, bloody, flying, fire-breathing lizard.
Classic image of high elves: noble, slender, beautiful, ancient humanoids.
Classic portrayal of magic: muttering magic words and *bam* magic.
I could go on. These are all wonderful—especially as fallbacks, little minor things to fill in at the spur of the moment, since they require no real explanation—but boring. If something classic is going to figure into your work, change it up a little. I once read an interview with Masashi Kishimoto (the writer and artist for Naruto) on how he came up with the world. He just said (I'm paraphrasing) that most of what he came up with was him just trying to break the norm. Ninja? Put him in an orange track suit. Magic? Screw magic words, they're just going to make pretty symbols with their hands.
Guess what? It worked! Huge, multi-national franchise from a blonde, orange ninja that makes shadow puppets (I love Naruto, don't get me wrong on that point. The world-building is breathtaking, to say the least.)!
So, why not have some fun and change it up? It's one of my favorite exercises. Take a basic idea like dragons and see where you can go. Sure, fire-breathing is awesome, but take a look at Douglas Niles' drackans from the Watershed Trilogy. They breathe a fog that knocks out their prey...until they taste darkblood. Then it turns into a scorching blast of steam. They have a carapace over their wings that breaks open after puberty. They have a color war between brown and green. I love them.
Dwarves don't always have to dig tunnels and have excess body hair. Who says there can't be comely, dwarves? Who says elves have to be gorgeous, or shoot arrows? Why can't an elf, just once, shoot a sub-machine gun, or be a heroin addict?
The best wizard I've seen wasn't just kooky and wise—he was just insane, rambling, and had no bloody idea what he was talking about.
We all know the normal things, but they've been done to death, revived, and killed again at this point. Yes, you are going to get crap from people for messing with the 'official' lore. Stephenie Meyer did, and she has one of the best vampire mythos I've read. But, you can also root into the foundation much better with something different—I bring you to Meyer again. In today's age, vampires sparkle and are very, very pretty.
So, take a chance and have some fun. Screw the norm, just do what you want to do—if you prefer that your dragons live underwater and knit sweaters for the fish, make it work.
Now go, frolic with your imagination,