Pseudonym
Ivan Alias
Planescape turned them into the D&D equivalent of Star Trek aliens, IMO. It made them far too human, made their cultures and their mindsets far too comprehensible. The notion of a demon or a devil sitting down for a drink in a tavern in Sigil (just for example) is absolutely anathema to what these creatures are.
I don't think I've encountered the Star Trek alien analogy before, but it perfectly summarises my main issue with Planescape as well. Demons and Devils tending bar and speaking Cockney slang really made it hard for me to appreciate the setting. For all its good parts, that just jumped out as viscerally wrong to me.
I'm willing to give the 4e treatment a go, however.