Vocal melodies and lyrics.
They're maybe the most accessible part of any song, and they're what stand out to me on the first listen. I've learned not to put too much stock in what I think of any DT album on that first listen, because it's the less easily accessible stuff that ultimately will decide whether I really dig a song or album. In almost all cases, I wind up liking DT's stuff far more after 25 listens than I do after one. The only exceptions to that have been WDaDU and ToT, for whatever reason.