Don't make promises you can't keep.
It's unbelievable how common it is for artists to send in music and include something along the lines of:
If you just give this a chance I promise you won't be disappointed!
Having confidence in your music is good. Tell us why you think it's good, how much it means to you, and what you hope other people take away from it. But don't promise us that we're going to like it. We could argue all day about what makes music "good" and whether or not "good music "even exists, but it's always going to come down to opinion.
Instead of selling yourself like some new sponge-mop vacuum cleaner on a 4 a.m. informercial, be honest. You can't possibly promise that we won't be disappointed in your music, but you can tell us why you think we'll like it.