I agree with what Fade has said above. His points are more regarding the technical aspect of DJing. I would start even more basic than this.
#1 - Picks the wrong songs/types of songs for the venue/crowd. (i.e. - playing EDM bangers while opening up at a lounge club, or playing 2 Chainz songs at a shopping spot with kids around) Having an understanding of where you are and what you are trying to accomplish and then choosing the correct soundtrack for that goes a long way. I don't care if you can't mix, but if you're the opener and you're playing bangers, I am not going to like it. (Unless the party is crazy and people need to wild out, but at 7pm that is not often the case)
#2 - Like Fade's #4, understanding energy changes as in building energy/choosing appropriate songs. If you constantly jump back and forth between smooth jazz, heavy metal, reggae, EDM etc... you may have a hard time having others follow exactly what you are trying to do. I'm not saying you cannot be eclectic, but understanding that a DJ set should have rises/falls and dynamic changes just like a song (only longer) can go a long way. (now playing a jazz sample and then going to into the hip-hop song that sampled it is not wrong by any means, just having a basic understanding of not jumping all over the map and having an ADD type approach to DJing is what I look for in a DJ)
#3 - If you are going to scratch, get an idea of WHEN TO SCRATCH. DO not just start cutting over Guru's second verse in Full Clip. Well produced hip-hop songs have scratches at the right time. Figuring out when that is in your set will make you sound much better. It gets too "busy" when a DJ is scratching over the top of a part of song where it doesn't belong.
You may already know a lot of what I have mentioned here, just a few basic things I could think of here. I will try to come up with some other ideas.