Skyfall. I personally didn't regard that film as highly as many seemed to, but I'd still agree that Spectre is a step backward. It's a quite workman-like action movie that does generally get the job done, but that won't likely leave much of a long term impression.
Actually, this movie isn't living in the shadow of Skyfall nearly as much as it's living in the shadow of the most recent Mission Impossible movie, Rogue Nation. The plot of this movie is quite similar. A long-running intelligence organization is closed down by a new bureaucracy, leaving the hero spy disavowed. With the help of a few rules-breaking allies, he works to bring down a massive conspiracy of evil that is actually pulling the strings of the replacement government organization. Add a dash of the "critique of mass surveillance" that The Dark Knight already covered well seven years ago, and voila! Spectre.
Familiarity didn't quite breed contempt in me, though. There are a decent number of good sequences throughout Spectre (even if you do have to sit through a sometimes slowly paced two and a half hours to get there). Most of the action scenes are quite fun, and depicted with reasonable realism by returning Skyfall director Sam Mendes. An extended fight inside a train car is particularly good for its visceral qualities, while the opening helicopter sequence is strong enough to leave you sated until the story has been set up fully.
As far as the cast goes, this is a film for the returning performers. Daniel Craig remains a worthy Bond (so long as you like your Bond broody); Ben Whishaw and Naomie Harris are both fun as Q and Moneypenny; and Ralph Fiennes does a lot with a little to dignify the proceedings as M.
The new characters get shorter shrift. After all the talk about Monica Bellucci being the first "older" Bond girl (Daniel Craig has said that she's "age appropriate"), she actually has little more than a blink-and-you'll-miss-it rolein the film. Christoph Waltz doesn't get nearly enough time to build an intriguing villain; after Javier Bardem was so well received in Skyfall despite not appearing in the first hour of that movie, the writers apparently decided to go for even less this time. Dave Bautista makes far less of an impact here than he did in Guardians of the Galaxy. And a minor, weaselly government bureaucrat gets far more attention than he probably deserves thanks to the casting of Andrew Scott, Moriarty from the BBC Sherlock series.
I think by the time I'm talking about a C grade of some kind, I'm finding a piece of entertainment actively bad on at least some level. For me, Spectre wasn't quite that -- it simply wasn't particularly special in any way. So I'm going to give it an unenthusiastic B-.