The trip hop is irrelevant -- my favourite Portishead album is actually Third, which largely left trip hop behind. And Beth Gibbons solo album from last year.

Mostly, it's a combination of interesting song structures, Beth Gibbons' vocals and lyrics. As Portishead evolved (and later as she went solo), it became more artistic and less poppy. And I like the later stuff more.

Still like the early stuff, granted. :)