It depends. If a sequel is less complex than it's predecessor it is most likely a failure. The exception is when the complexity was a very negative thing but I can't even think of an example where that was the case. Most sequels these days change a few things in the gameplay, have some new levels, a couple of new characters and a new plot. That isn't enough for a great sequel. The ratchet and clank series is a good example of a series that is complacent with their formula. Since the 3rd iteration, they've simply palette swapped things like weapons and the puzzle minigames, had negligible plot development, no new memorable or significant characters, half-assed innovations (the time puzzles were cool, but why only give us 2 interesting ones? It's like portal ending on level 5) and they really lack the sort of depth that the audience is looking for. On the ps2 they made online multiplayer. On 3 sequels on the ps3 where multiplayer has become far more prevalent, they have not attempted it once. A r&c multiplayer could be a great change from more realistic shooters.
Shallow games are fine for nintendo platforms, but playstation (and xbox too) gamers are into grand theft auto, final fantasy and metal gear solid so we expect at least an attempt to tell an engaging story as well as improve things. Final fantasy is in a new world with new characters every game. GTAIV added many things like the very complex internet parody, dating and real-time physics. MGS4 added a remote control robot, octocamo, the SOP system, the best 3rd person control scheme ever and the greatest multiplayer shooter ever in terms of depth and the balance of skill and teamwork.
New Vegas disappointed people for using the same engine with no improvements, but what's wrong with that when the engine is so good to begin with? Not in terms of polygons and textures, but radiant AI, emergent gameplay, 2300 persistent characters, 150 locations, real-time physics etc. It offers a different experience from anything else and is vastly superior to the Crystal Tools engine which everyone loved because of high polygon characters (though they were at the expense of environments which are pulled straight from the ps2 era) despite having even worse animations and no physics system at all.
FFXIII is the worst sequel of all time. The series has been losing complexity since VII, with only the transition to 3d maps and the addition of voice acting adding to it, while they lose everything that made the series unique like towns, minigames, puzzles, exploration and interesting objectives. Now it's just run to the end of path while battling then watch a cutscene. That's ok if the battle system or cutscenes are great, but they were horrendously bad.