Most young writers forget that the whole process of writing should be a learning experience. If you only write about what you know and your characters have the same characteristics and behavior patterns as yourself, you won't be able to write more than two screenplays in your life.
As a writer each character you create should be as different from you and as different from each other as apples are from zucchini. When you write a character you must know everything about him/her - where they were born, family history, illnesses in the family, scars, personality, education, etc. etc. Even if it's not part of the story you're writing, it is very important to know exactly where your character is coming from so they can relate better to each other and in case you write a sequel you have something to fall back on. As an example you can watch Back To The Future to see how characters were created with enough backstory so when it came time to do sequels (which there were no plans of making) they had no difficulty writing parts 2 and 3. If the writers didn't create solid characters with a rich background it would've been very difficult to write the sequels let alone tell a solid story.
In the relaunch of the Star Trek movie franchise, there is a large part of the plot which is based on the Kobayashi Maru test - for those of you who are unfamiliar with this test - it was actually first mentioned in Star Trek II: The Wrath of Khan in the original film series. The new Star Trek would've been a totally different film if there were no mention of Kobayashi Maru in the original series. The writers of the new film did their research and due to the efforts of the backstory put together by the writers of the previous series they were able to tell their own story and paid homage to the old one.
What I really notice with today's generation of writers, especially in Bollywood, is that they get very greedy with a joke. They're so busy trying to come up with the perfect punchline that they don't realize that they've revealed a part of the story during the joke which shouldn't be revealed 'till later on. I know it's the season of comedies in Bollywood, but one must remember that you can hit the punchline without revealing your story, the problem is no one seems to care and this is why a film fails to find an audience. The same is true for mysteries, writers are so greedy about adding twist after twist that the audience figures out the story in the first 10 minutes and the rest of the film is just dull and boring. The best mysteries are the ones which take you on a ride and have one twist at the end - no one likes to be on a journey with a thousand stops. A good example is the film Saw, while watching it you have no clue where it is headed, and then at the end... BOOM! There is a wicked twist which makes you say "that was incredible".
It's not difficult to write a story, but it is very difficult to write a great story, primarily because of the greed that most writers today suffer from. If a writer can put aside the idea of writing a hilarious comedy and just writer a story with funny situations and interesting characters he will find that he has, in fact, created a hilarious comedy. But when you set out to write a hilarious comedy you're so preoccupied with the perfect punchline that you forget about where the story is headed and end up with a horrible story. A great story can only go in a limited number of directions but when it gets pulled into unfathomable situations that's when you lose your audience. Your audience will forget and forgive a bad joke in a great story but they will never forgive or forget a bad story even if it had great punchlines...