Melodies: The main riff is pretty nice! It definitely merits being repeated throughout the song. Good job! My main criticism is that when you try to do variations or solos on it, you're using too many quarter notes and you need to mix up your rhythms a little bit. 1:54 is a good example. Or 2:19. More complex rhythms can spice up a melody nicely. (I've actually been thinking about this in context with my own music too.)
I like the break at 1:43. I thought the song was over for a second! haha. I think it might be more effective if you didn't bring in the bass immediately. I do enjoy how the song is empty for a second though.
The mixing sounds pretty good to me! The bass stands out and the other voices are not fighting to be heard. The only thing that I thought could be fixed was the drums. They're too quiet! EVERYONE gets drum loudness wrong, all the time, always, including me, even though I already know this - so don't feel bad. Slap a compressor on those bad boys. It doesn't have to be aggressive, but I do think they should be louder.
Synths: I think they sound cool, but I don't really think that they belong in a song like this. Like the first synth that you introduce (:18) has got this really eerie feel to it that clashes with the otherwise chill and happy vibe the song has going. I honestly think that a piano would be perfect. Though I always think a piano would be perfect. :D Other orchestral instruments to carry the other voices (like strings, woodwinds especially) would do well, too.
While I'm talking about sound choice, the drums are kinda electro and a clap is pretty weird in a song that's emotional like this one. I would personally use someone like Skullbeatz on newgrounds as a reference. He has beats with emotive tracks all the time, and he does pretty good at it. (I mean, come on, he has the word "beatz" in his USERNAME).
I'm all for using reference tracks. Good artists copy, great artists steal, blah blah.
I was also thinking that you could switch up the progression somewhere in there and get more feels out of the piece. The main progression is great and carries the feel of the song nicely but if you switch up then you can get even more mileage out of it when you switch back to the main progression. One that I think might compliment your main one pretty well is d minor -> g major -> c major -> a minor. Bonus points if you do the descending baseline C -> B- > A for the chord change from C major to a minor. It gives it that extra emotive feel. :D
Also last thing, don't be scared to loop your main melody a little more, you don't have to always make variations on it. Good melodies deserve to be repeated!
Overall, I like this song a lot. Good job! I think the biggest thing to focus on would be synth design. With more organic sounding instruments that match the feel of the piece, the song could go far! :D