Well, for me this is different every time really, it depends on the kind of music. For metal in general, I think mixing is one of the first things that tell me whether a song sounds promising or not. However, sometimes this can be deceptive, as I greatly underestimated Reinxeed and overestimated Innerwish for example.
Also, when I hear a song for the first time, surprising details like a certain guitar riff, drum fill or admosphere start to pop up and give me a reason to like the song. After listening multiple times to a song, I usually focus on every separate instrument and then how they act together. This is when I decide if a song is worth listening to for its musical quality, lyrical cohesion and talent and technicality of the band members.
Let me take 'A Nightmare To Remember' by Dream Theather as an example, which I recently heard it for the first time. At the first listen, the first drum fill really caught my attention, as well as the beautiful melodies during "In peaceful sedation I lay half awake". The mixing was appealing, nicely done, nothing more. After a couple of listens I started to notice how well the band members handled their instruments, and how closely the music was related to the lyrical theme.
I don't know how other people develop their (dis)taste for certain song, but I'm sure this is one of the more interesting features of music.