There are a handful of great trip-hop albums, and then nothing else. It'd be nice to see another great album added to the canon.
No reason Portishead should be limited to just doing trip-hop, though. Have you heard PJ Harvey's new album? It sounds absolutely nothing like anything else she's done.
Ah trip hop. A bullshit label concocted by British journalists to make downtempo electronica palatable to a mainstream audience that thought everything electronic had to sound like Kraftwerk. Never had any coherence, and never will. All the artists labeled so went on to do other stuff because they were never really doing trip hop in the first place.
But if you want to find catchy downtempo electronica
mostly with vocals, you could do worse then check out:
-- The Verve Remixed album
-- DJ Krush (particularly Meiso -- on Mo' Wax)
-- UNKLE (particularly Psyence Fiction -- also on Mo'Wax)
-- De Phazz (particularly Death by Chocolate)
-- Orbital (particularly the Blue Album)
-- Supreme Beings of Leisure (ah, Golddigger, the Crazy of 1998)