For me it's more of a total package than anything.
I think my initial excitement comes from the Theme. If I dig the theme, I get excited about the game even before it's revealed. That can change however if I feel like the artwork just doesn't do the theme justice.
From there it comes down to Gameplay, Code, Dots/Visuals on the LCD if applicable. There have been games that have a great theme and have a great artwork package that I genuinely think that I'm going to like but when it comes down to actually playing it, it just doesn't do anything for me. In that same manner, there have been themes that I was kind of indifferent towards, but the Gameplay convinced me of what a great game it was.
If I can find that happy medium between all of those, there's a good chance that I'm going to do what I can to add that game to my collection.