Harmonies flat
Good choice of song -- it's one I plan on doing myself.
The synth notes sound a bit flat -- the harmonies suffer from it, as one part that is slightly off interacts with another part that is also slightly off in a different direction, making the final mix sound soured. Try tweaking the sounds ever so slightly and spend the extra time going over your mix with a fine-toothed comb to create a more polished finished product.