Specific spots of trouble:
1. I find periodic filter sweeps to work best BPM-synchronized when there's a rhythmic reference in the background--in this case, your drum loop.
2. Noise which sounds like a dying buzzard is horribly grating on the ears. Filter out those high frequencies.
3. Bass drops at 0:21 and 0:36 seem pretty random and out of the blue.
4. Ending was rather abrupt.
General criticism:
1. Song doesn't go anywhere. There doesn't really seem to be any sense of structure or progression, or... anything, really. It's like 3 different things cut up and stitched together.
2. FL stock voice samples. No. Don't bother adding vocals just for the sake of having vocals. If you really want vocals, you're going to have to either have them recorded or rendered yourself, or build your song around them, not try to find a set of vocals which clashes the least with your song.
3. Bassline doesn't go to anything.
4. Goes with 1, kind of. You don't want to suddenly change the music without any sort of warning. Add some sort of transition between different parts of your song. Example: 0:46 when the horrible buzzard death-cry cuts out suddenly, and at 0:54 when it reappears just as abruptly.
5. Consider buying a book on music theory, or browsing around online.
http://www.musictheory.net , say. Ignore if you're trying to make atonal music. If you're not, your song lacks a sense of underlying harmony. If you changed the pitch of any instrument by half a step, you wouldn't make an appreciable difference.
Not a complete list, but what I could come up with at the moment. There's definitely more advice people can give you.
