I think the main story portion of Final Fantasy XIII, which takes maybe forty to fifty hours to complete, is great. The graphics are gorgeous, the level of attention to detail is amazing, the way in which the narrative threads come together is fascinating, the battle and leveling systems are interesting, and the linearity doesn't actually bother me that much, all things considered. I also like the world, the character relationships, and the main themes of the game.
After the main story ends, however, the developers tried to drag the game out for another forty or fifty hours, and the two primary methods they use to do so are so artificial that they're painful.
First, instead of giving the player more areas to explore, the game only gives the player more enemies to fight. These enemies are very powerful but not necessarily interesting. Your party does the same thing that your party has been doing for the past four dozen hours, except now, instead of a major battle taking ten to fifteen minutes, each major battle takes at least half an hour. That's, again, at least half an hour of buff-heal-attack buff-heal-attack buff-heal-attack for every battle against a palette-swapped enemy with a huge health bar. Some of these enemies don't get interesting until the player has already invested forty or fifty minutes into the fight, at which point the enemy flips out and kills everyone, game over. The worst thing about these fights is that, if you win them, they don't reward you with anything except bragging rights; they don't really help your party get stronger.
And that's the second thing. There is absolutely no way for your party to get stronger after the game except through dozens and dozens of patient hours of grinding. The ratio of time invested to rewards earned is miniscule, and there's no way to speed up the process. I don't want to go into the details of how this process works, but I could probably study for and pass the bar exam in the time it would take to power up a party in FFXIII to the level the game demands to fight some of its post-story bosses.
FFXII also had a lot of monster hunts, but what was great about these hunts was that (a) they required exploration, (b) they required strategy, (c) they rewarded the player with tons of neat spoils, (d) they rewarded the player with additional narratives and artwork and world building, and (e) the player didn't have to wait until after the game was already over to undertake them. Also, if your party wasn't strong enough to win against one of these monsters, the game let you know right away instead of stealing an hour of your life. FFXII was a long game, but it never felt artificially long, and it was always up to the player to decide how quickly she would move through the story.
In conclusion, I think that maybe FFXIII could have been a better and more satisfying game if its developers had been content to let it be what it was without trying to artificially expand it by making everything after the story take forever to do. Sometimes a game "only" takes seventy or eighty hours to complete, and that's okay. Not everything needs to be a MMORPG, which aren't really that fun when they're single-player anyway.