It doesn’t have to be complicated.

I won’t lie, I am a fan of complex music.  My favorite band is The Dillinger Escape Plan, and they are quite complex in the songs they write and play.  I dig that, but sometimes it’s best to have something simple.  Back in the 80s, some of the best bands around didn’t do anything complicated, like Black Flag and Minor Threat.  The 90s with Blink 182 and No Doubt (though ranking them alongside Minor Threat and Black Flag is borderline blasphemy).  Today, people are trying to make complicated songs with a whole lot of different things going on in them, and while it works for some, it doesn’t work for others.  Often, bands add in “parts” solely for effect, “let’s add a mosh part here, a dance part there, a melodic part there.”  After a while, people see through that and notice formulaic bands.  If you’re worried that people will lose interest, write better material or cut the song short, sometimes it doesn't help the song to add a mosh part for good measure.