How To Increase The Fun Factor of Your Fiction
1. Put in some cool stuff that does cool things.
Remember wanting your own magic wand, lightsaber, or TARDIS? Or maybe your own enchanted ice sword or a power item that transformed you into a magical girl? How about your own alchemiter? People love having cool toys to play with; indulge them now and then.
2. Include places your reader will want to visit and explore.
For example, the world of Harry Potter is full of places that capture the imagination, from the shopping district of Diagon Alley (full of strange and wonderful items to buy!) to Hogsmeade (who doesn't want to go get a butterbeer at the Hogshead?) to Hogwarts itself (it's chock-full of secrets and surprises!). Doctor Who treats fans to all kinds of strange and wonderful worlds.
3. Fill it with fun things to do.
EG, holidays, sports, games, and traditions. The best example I can think of is Harry Potter, which gave us Quidditch (a sport played on flying brooms!), absolutely magical holiday parties, and of course, the Yule Ball. Recently, My Little Pony: Friendship Is Magic has introduced several fantasy counterpart holidays (eg, Hearth's Warming Eve, Hearts and Hooves Day). When I was a kid, I wanted to play on the Holodeck in Star Trek SO BAD.
4. Make at least part of the main characters basically nice, friendly people you'd want to hang around with.
They don't have to be perfect, but at least some of your characters should be the type of people that you'd actually want to have around with you in real life. I don't mean the type of people you'd fantasize about having around you in real life, eg, glamorous movie stars, your own personal valet, or some rich person who will share xir goodies with you, but rather the kind of person whom you could actually expect to be friends with in real life. For example, I think most people would probably want to hang out with someone like Applejack from My Little Pony or John Eggbert and Jade Harley from Homestuck.
5. Use a variety of character types.
I was originally going to title this "fill it with interesting characters," but honestly, just about every character type can be interesting to someone. Instead, one thing I've noticed about worlds that looked like fun to be in is that the primary and secondary characters embody a wide variety of character types. In The Avengers, for example, Tony Stark is the geek/smartass, Bruce Banner is the brooding genius, Thor is the stoic warrior, Steve Rogers is the guy you'd bring home to Mom, and Loki is the bad boy. During the run of Lost, in which main characters frequently came and went, each character was unique and had his or her own quirks. This assortment of characters not only ensures that interesting things can happen when they interact, but it also increases the odds that people will find a character they identify with or find attractive.
6. Have fun with your characters.
Don't be all dark and angsty all the time - make sure your characters have fun and do fun things. Non-stop drama, angst, and serious business gets tiresome and even depressing after awhile.
7. Don't over-grimdark it.
There are some people out there who just don't seem to be content unless everything in a work is presented as dark and morbidly as possible, whether or not it was ever dark and morbid to begin with. (Supernatural is a pretty big offender here.) While I enjoy a good subversion, I feel there's such a thing as going too far, especially when the story starts taking a decidedly mean-spirited tone about it, EG, treating characters (and by extension the audience) as a bunch of pathetic ignoramuses for ever thinking that something could be nice in the first place. Let's say, for example, that you're writing a fantasy. Sure, go ahead and write in man-eating fairies. Reveal that Hansel & Gretel was based on a true story about a child-eating demon or something. But maybe have leprechauns turn out to be all (mostly) all right, or unicorns are a real and generally awesome thing.
Also, check out:
Tropes Used in Successful Fantasy & Speculative Fictions That Inspire Creativity in Fans