Kirby's art was such that an inker could seriously affect the finished product - look at how Kirby's pencils popped when a Joe Sinnott fleshed them out.
A lot of his stuff post-Fourth World was inked by Mike Royer, and in my opinion, Royer probably wasn't the best embellisher to bring out Kirby's strengths (witness the square-off fingers, etc.).
Kirby's characters and stories were never again as brilliant as what he contributed to the classic Marvel era - but, to be fair, you'll notice that Stan never wrote anything brilliant after he left, either.
