I agree with Gario. The piano sticks out the most, IMO, as it's both mechanical and reverb-less. The instruments are also all competing for attention. It sounds like all velocities are completely flat, and although synthesizers aren't necessarily expected to sound realistic (unless it's a synthesized supposedly organic instrument), having natural velocity variation would help them to not sound machine-gun.
It may be a good exercise to learn a little piano so you can get some insight into how velocities might naturally flow. If you pay attention to how heavily your fingers press on a keyboard for each note (and it is not exactly the same nor negligible in the differences for each finger), you should eventually be able to think about it while you're sequencing and do velocities while you're writing. Note that when I say "not exactly the same", it implies the velocities should be different, but when I say "nor negligible in the differences", it implies the velocities shouldn't be too drastically different (otherwise it would be "pulsing").