percussion and vocal
There seems to be a "vision" behind this music, but it is just out of focus...taming your timing and pitch will improve it 100 percent...
unless you are going for some kind of "no-drum" concept I suggest (if money is tight) that you find a Casio keyboard or a guitar pedal (there are $70 pedals that have built-in drum patterns) that will furnish you a consistant beat with which to play...a repeating drum pattern can sound monotonous but keeping it back (lower) in the mix will help prevent that...if you ethically can't allow an electronic drum pattern in your song prehaps you can listen to a drum pattern (or metronome) in your headphones or on a scratch track until the song is finished and then ditch the drum track...it seems that the quitar and bass are not anchored down, a drum beat would force them to be diciplined and "honest"...you could more easily hear which instruments are "out" and get the music into a healthy rythmic feel...
as far as singing: I believe vocal ability consists of hardware and software, some people have great tone (hardware) but have lousy timing (software), other people can mentally hear what notes they need to hit (software) but when they try to hit those notes their vocal chords (hardware) just don't cooperate...the problem with this vocal seems to be "software" related, that is, the tone of the voice is acceptable, but it seems to go out of pitch and makes for some bad moments in the song...the keyboard exercise mentioned in another post might help this problem...If you know someone who is generally believed to be a good singer you might have that person listen to you sing each line, have the (teacher) vocalist sing the line correctly, and then repeat the line back to the teacher, etc (I'm just brain-storming here)...my point is:keep trying to fix the software, keep practicing, listen to good singers, listen closely to recordings of your own singing and be open to advice from others, do not be stubborn and just conclude that it is the rest of the world that is wrong (as you might conclude, I have issues when it comes to this subject)...it may take years...but you will either get better or conclude that you lack in the singing department.