Games get bad when they feel linear. For example, when whatever choice you get is in vain. Ever played an RPG where you get to answer Yes or No to a question, but if you choose No the question just repeats until you say Yes? That gets on my nerves (Zelda for example does this all the time... why even bother giving me a choice?).
Also, First Person Shooter games nowadays are far too linear for my taste. It's mostly just a single corridor you can go down on, and any objective is reduced to stop and press a button, or stop and fight some enemies. It get's very repetitive since 99% of all FPS today are designed like that. Some games, like Half-life 2, manage to hide the fact pretty well by using pretty enviroments that seem open, but most games don't even try to conceal it, like Quake 4 or Prey. It all reminds me of old sidescrolling platformers, your only mission is to go from left to right. It kind of defeats the purpose of having a game in 3D if you progress in a 2D fashion, doesn't it?
Compare to older games like Doom 2 or Duke Nukem 3D, those games were linear but level progression was not so restricted, you could travel down several different paths in a level, finding keys and items, eventually reaching the end. Nowadays, it feels like FPS have gotten dumbed down, and considering how straight forward the genre has always been, that's saying something.
Linearity is good to a point. I don't mind having just one ending, or not having multiple choices all the time. But when FPS play more like rail shooters than actual 3D games, it gets on my nerves, and that's when I start critisizing.