If there's one thing that doesn't get enough attention and in my book is THE MOST important to getting a great end result, it would have to be arrangement. Not only the song structure, but what instruments are paired with other instruments, the timbres they provide to the sonic structure of the song, where they're placed in the stereo field, the melodic and rhythmic decisions made for each instrument and last but certainly not least...the space between each note. These are the next biggest deciding factors to if you have a great song or not. These make the job of mixing and mastering much easier and are where the majority of your time should be spent.
We often forget that some of the greatest albums were made on the most primitive of technology, from The Beatles at Abbey Road to the recordings of Count Basie's Orchestra to Miles Davis "Kind of Blue", if you focus on these first three things you're bound to make a more professional sounding recording even if you don't have the highest quality recording gear.
"It's not the notes you play, it's the notes you don't play"--Miles Davis
