also it sounds cheap when instruments are used in a way that they don't sound natural...what i mean is like a saxaphone player doenst control his saxaphone with keyboard keys...so you have to remember that and play it (as much as the keys will allow) like a saxaphone (or trumpet or whatever)...this might mean picking different sounds as your main melody and using the real instrument (horns, strings, etc) patches as accents (or just sample some horns, strings, etc). also, learn some piano chords...play those with your strings and it makes them sound way thicker and more realistic.
and...reverb doesnt mean automatic realism. sometimes it sounds cheaper...like formant said, its all about balance and learning how to best use your patches to fit your style. from the stuff ive heard it seems like you are really melody oriented...which is good but that also requires you to get that much more acquainted with your sounds on that motif, ya feel me?
hope this helps