I'd like to point this out specifically because I think this particular point is tough for me; at least on my track for this round specifically, I've received some feedback that some of the 4 elements (lead, accompaniment, bass, & percussion) things overlap in frequency range, making it hard to clearly discern all the parts. Production (in terms of EQ) can really help with this, which I actually did a lot of on this track, but the best thing you can do to prevent this problem is to choose patches/samples/what-have-you that don't interfere with each other too much; i.e. like Darke said, don't make your lead so low that it's in the same range as your bass. This overflows to the actual structure of your patches/synths as well; if the notes of your melody occupy the mid-high range but the synth itself exists in the mid-low range (maybe because of harmonics or overtones or resonance or whatever) then it will interfere with whatever else exists in that frequency range. Again, EQ can help with this but it's better to *start* with patches/whatever that you know will have little to no frequency overlap. And actually, this can occur with your drums as well - your kick can have too many low-mids, or your snare too many high-mids, etc. or your other sounds could have too may of those frequencies and it'll all make a big mess.
Again, this is something I often wrestle with so I'm preaching to myself here