In casual usage, the definite article is often omitted at the beginning of a statement, sentence or utterance. It should only be done where there's no ambiguity created by the omission. It's also usually used to contrast a previous statement, from the same speaker or not.
Also, all the cases I can immediately think of where I would expect to see are for things of the form "The <noun> is..."
Trouble with you is that you're always thinking of the downside
Upside is that we get free drinks
Nice thing about Newcastle is how friendly everyone is
All of those are things I wouldn't blink at, though by and large the negative cases, or ones expressing caveat, are much more common in my experience. So that's things like:
Thing is I can't just jump in the car.
Problem is we can be sure she'll turn up.
However, in all of these cases, Standard English requires the in front of it, and you should avoid it in anything formal. Only exception would be (see what I did there) if you were trying to create a light, conversational tone in your writing/speaking - which is sometimes desirable.
Familiarity and experience are the best way to judge when and how to use things like this.