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Final Fantasy XIII had some pretty serious flaws. It took ages for the battle system to open up, and much the game was stupidly linear and I can totally appreciate why those things would turn people off.
But far too many people act like it has absolutely no redeeming qualities whatsoever and I find it hard to take those people seriously. Sounds like a huge case of butthurtedness and disappointment to me. "Wah, this isn't Final Fantasy (insert favourite here) again! Worst game EVER!" Well, different numbered Final Fantasy games are different. Deal with it.
Take the battle system. It's so good! Easily the best battle system in the series. Fast paced with lots of room for strategy without ever expecting you to micro manage everything. I love setting roles instead of issuing dozens of individual commands. The party AI is usually really good, trying new things against different enemies and finding what works. (There are a few hiccups now and then, like casting En-element spells on party members who don't use physical attacks. :P) The Stagger system is great, too.
Gran Pulse was huge a had lots of places to explore and tons and tons of sidequests to complete, with lots of tough enemies and bosses that gave the battle system a reason to live. The story was no worse than any other JRPG, seriously guys stop pretending Final Fantasy VII a great story, it's embarrassing.
I can't really fault anyone for not wanting to sit through the first 20 hours of the game until the character development system and world open up, and the complaints about the pacing and linearity and limitations on the various systems for the first half of the mandatory story part of the game are certainly valid, but come on. If you seriously can't find a single good thing to say about Final Fantasy XIII, you're clearly missing something.