Gargantuan bosses that look impressive in scope, but are beaten through very easy means, often times through a quick-time event.
For example, look at the Titan from Resistance 2. It's a huge, imposing creature. How do we beat it?
Step 1.) Run down linear sequence of paths (I don't think I even needed to dodge anything, if I remember correctly)
Step 2.) Pick up rocket launcher
Step 3.) *Boom*
For another example, think back to the Guardian boss fight in Darksiders 2. Sure, he was really big, and the music was spectacular, but could you really call it a challenging boss? Was finding out the method to beating him in any way difficult, what with his glowing weak points? (Note that I still enjoyed these two games, it's just that these boss fights were major disappointments for me, as they are in any game that handles them in a similar fashion).
It's gotten to the point where I don't even get excited when I see a large boss anymore, because they're usually beaten in such easy ways, and are made almost purely for the spectacle of "Oooh, look at what our graphics engine can do!".
Shadow of the Colossus, however, did it right (thank goodness for that, since it's the entire point of the game). Yes, the bosses had glowing weak points, but you had to discover them for yourself, and finding the methods to beat them and planning the best climbing routes while managing your grip gauge was actually a challenge.