The first cardinal sin of DJing is bad sequencing, and it is by far the worst. If you don’t play the right tracks in the right sequence then nothing can save your set.
The second is playing a prepared set. Really, why are you there in person if you can’t react to the situation?
The third is not being true to yourself. There are occasions where playing music you don’t like may be required. They’re called weddings. Do so anywhere else – and it’s high time to change what you are doing.
The fourth is not having any self to be true to. If there is no music you love then how can you expect your audience to love the music you’re playing?
The fifth cardinal sin is doing stuff you can’t do with absolute certainty. If for example you can’t properly beat match then do quick cuts (or use software that does the matching for you for God’s sake).
The sixth is doing too much. The chances that you know how to materially improve ever single track in your set through your scratching, filtering and other antics are slim. Trust your material – if it needs that many modifications then you should really play something else.
The seventh is not exploring new music. Not necessarily music that is newly released, but music that you haven’t heard before. New music forces you to shake things up, try new sequences of tracks, and keeps your sets fresh.