I break it down into 4 categories, which I score out of 4:

• Mixing: How well the song is mixed. How well each instrument stands out. Hopefully there are no instruments with overlapping frequency ranges. Is the mix full when it should be? Do the drums provide a good backbone, if there are any?
• Composition: The melodies and chord progressions. How good are they?
• Arrangement: Does the song keep my attention? Or is it too repetitive? Does it change and evolve? Do the energy levels change over time?
• Sound Design/Sound Quality: how good/realistic do the sounds that you are using sound? 

My overall score for your song has nothing to do with these mini-scores. All it has to do with is how good I think the song is. I provide the mini-scores as a way for you to know what to improve.

Q&A:

• How can I improve my mix? Read this. Additionally, if your song sounds too muddy, you have too much reverb. 
• How can I improve my composition? Learn how to play an instrument (ideally piano), then play along to the songs that you like. Especially try to break down the stuff you're not familiar with, like weird progressions or fast melodies.
• How can I improve my arrangement? Er, I don't have a guide on this yet, sorry. Change stuff up every 4 bars - add/remove elements, and try including B/C sections as well. Also try modulating. Arrangement is probably the easiest of these 4 to get down as long as you vary stuff every 4 bars.
• How can I improve my sound design? I also dont have a guide on this either. It's actually "easy" though - listen to your favorite songs and pay very close attention to the types of sounds they use. Make a habit of this; do it every time you listen to music. Try to recreate similar sounding sounds. 

Hopefully that helps! Good luck, everyone!