Songwriting is number 1 priority for me; melodies and hooks that stick in my head and make me want to play the album again. There are tons of four-star bands/albums out there, but to achieve a gold pedestal in my hallowed hall of fame requires superior songwriting.
Key factors for me are monster riffs and emotional solos, and no growling/grunting/barking from the mic stand. Tempo is also a factor. I don't mind speed metal tempos if used sparingly, but if a band bases their whole delivery on how fast they can play, I get bored. My philosophy is you can only play so fast before it ceases to be music and just becomes noise. Same with extreme vocals; you can only get so abrasive before you cross the line between music and an intentionally silly Sesame Street muppet.
That said, superior songwriting can negate the pet peeves I have in metal. Examples are: early Metallica and Megadeth riffs overpowered the shitty vocals, the brilliance of Orphaned Land and Devin Townsend force me to accept growling despite my dogged resistance, and catchy songwriting keeps me coming back to Dragonforce's Inhuman Rampage despite it's over-the-top speed and repetition.