It just goes to show how different people view things differently. I LOVED FFXIII. I loved FFX too, and most FF games I played. XII was my least favorite, due to the poor combat system, leveling system, and items. XIII is easily one of my favorite FF games. In fact, I think it may be the most fun I've had playing an FF game.
FFXIII Pros:
Great graphics, videos, music, production values, etc.
Fast paced combat. This is the fastest combat of any FF game, and it really made it exciting for me. It was the most fun and excitement I had playing an FF game, such that even trash was fine to fight through.
Hard battles. I died more times in FFXIII than in any other FF game. In fact, the only RPG where I died more was Demon's Souls. There were random monsters wandering the world that could literally squish me flat, it made for a nice sense of awe.
It doesn't waste my time. When you die to a boss, you don't have to re-clear an hour worth of trash, you respawn 10 seconds before the battle. This lets you get right back into battle with a different strategy, different gear set, etc. I don't have the free time anymore to glibly lose 1-2 hours of playtime, and I appreciate that the game understands that.
Actually using your items / equipment. In most FF games, you don't know what's coming until you get there, so you never use the "Absorb X" or "Half damage from Y" items you get, you just load up the generic items that are always beneficial. In XIII, if you come across a fight that's too hard, you respawn, load up on the right gear, and do it again. In every other FF game, you power level to the point where you never come across a fight that's too hard, since dying means losing an hour or two.
XIII Possible Cons:
It's linear, which bothers some people.
It has few side-quests, which again bothers some people. They're all of the go and kill X monster variety.
It doesn't unlock the full party setup / customization options until 75% of the way through the game. I felt it worked well, forcing me to use setups and characters I otherwise wouldn't, and really give me a sense of the different classes. But again it bothers some.
It doesn't have much in the sense of item collection or equipment, which makes sense given the game world, but obviously doesn't have the loot whore appeal of diablo.