1. Yuffie steals materia: Nobody's perfect, but when Yuffie steals stuff its just stupid! Of course, some argue it makes up her character nd make her 'different' than most chracters but if stealing makes up who you are then you have problems. In FFVII Yuffie repeatedly steals things from you, beginning with the second you first meet her. She starts by taking a crap load of the gil you have and if that's not bad enough it escalates into something far more devious and utterly dispicable. If you answer her questions 'wrong' in the game (namely by basically telling her to screw off/go away or not expanding her already HUGE ego) she will steal your gil, and more of it if you encounter her and 'correctly' answer her questions but then answer one of them 'wrong'. Some would also agrue that later in the game gil is really easy to earn back and it makes no difference, but its the fact she STEALS from you that is the driving reason to why she needs to be taken down a peg or two - whoops I'd better make that a million pegs instead. In the game when the characters journey to Wutai, Yuffie steals ALL of the materia you have in your slots and forces you to go on her waste of time sidequest so you can get them back. Not to mention the fact she hides out around Wutai and when you open one of the chests in the weapon shop she steals what you found inside it - and that is so annoying it makes me sick!!! How can someone who steals stuff from their own friends be cool? Would you like it if one of your friends purposely stole something very important from you and refused to give it back? Face it, in real life Yuffie's ass would be sent to prison for shoplifting and she'd be serving numerous sentences since she can't stop. You can also assume she is a kelptomaniac because of her constant stealing habits, but I wouldn't give her that kind of sorry diagnoses in replacement for pure greed.
2. Yuffie has an ego issue: That's right, and its plainly obvious by that little smirk she constantly wears on her face as protrayed in several of square's picture artwork. Arrogance can be a cool thing if it is protrayed through a cool character, but someone like Yuffie just makes cool arrogant characters look bad. Worst of all is that she always brags about how she is the best ninja?/thief ever and how everyone should be afraid of her (ie: when Cloud first met her she assumed he'd be scared of her, but most likely he wanted to kick her ass). Yuffie uses her arrogance to try and prove to other characters how much better she is, when really she is just proving how much of an egotistical loser she is and how she has to drag down/annoy the rest of the team to attempt to make herself feel better. It is known that many hate Cait Sith since he is considered similar in attitude, however he redeemed his ways by sacrificing himself so Cloud could get the Black Materia out of the Temple of the Ancients (how disappointing though they didn't let Yuffie stay in the temple when it collapsed to pieces...) What has Yuffie done to redeem herself, or perhaps give a hint she isn't such a waste of time after all? NOTHING! Absolutely nothing at all! There is that little possible date scenario with Cloud and Yuffie that could be made possible in the game that may have proved Yuffie does have a nicer side, but not even that gave any inkling she was anything other than a materialistic and selfish stealing brat (not to mention Cloud didn't really want to go with Yuffie in the first place, and that violent ninja slapped him!)
3. Yuffie Is a Poser of Aeris and her limit breaks suck: It must sound crazy to say that, but I don't mean in personality is Yuffie like Aeris. One of Yuffie's limit breaks is called 'Clear Tranquil' which is suspiciously similar to a limit break of Aeris's called 'Healing Wind'. If Yuffie doesn't even have the originality to have limit breaks entirely unique to her then it only proves the point she steals a lot more than gil and materia - attacks as well! Yuffie's limit breaks are probably the worst in the game because they hardly do any damage, even when she is at a higher level they still do the minimal requirements in the game. That simply reduces the reason why she should even BE in FFVII to begin with, because is her attacks were at least descent then it would make up for something but she doesn't even have that saving grace!
4. Yuffie's personal problems aren't even problems at all: In FFVII, Yuffie constantly whined about how Wutai is turning into a tourist town and not focusing enough on being a Ninja place where people are far more noble and etc. For one, what's wrong with tourism? It brings economy into places, promotes cultures and ideas and shows off what a place has! If she thinks Wutai is such a great place, then what's wrong with having other people come to see it to prove it has things to see and do that make it great? Technically she doesn't own that town, so anyone has the right to go there and take a look if they want to! Who does she think she is to say visitors aren't allowed, what does that make Wutai anyway some kind of ninja only club? Please, the other characters in FFVII had way better problems to sort out that were far more significant and deep than Yuffie's. Cloud had emotional/psychological issues concerning his past, Sephiroth was very upset because of Shinra, Barret lost his wife and best friend to the greed of Shinra, Aeris was orphaned and experimented on and then killed and Cid had his rocket program completely dumped and was shafted by Shinra. Those problems are a hell of a lot worse than some issue with tourism in a town!!! If you argue everyone's problems are different and some are more significant to one's own life than others, then its plain that Yuffie's problems aren't really her problems at all. What if other people living in Wutai don't feel the same way about the tourism? If it is a problem then its the town's problem and not just hers (but since she's selfish anyway she automatically thinks its her job to 'right' what is supposedly 'wrong' even though nothing is wrong with her town to begin with other than she is the problem and perhaps maybe people in Wutai should consider getting rid of her instead of the tourists).
5. Yuffie is often paired with Vincent in FF Fanfiction: That may not be an official reason from the game's perspective, but I've read far too many poorly written 'Yuffietines' to sit quietly and not say anything about it. Its about time some people faced the fact Vincent is so undoubtedly cool that anyone such as Yuffie certainly doesn't belong with him. Vincent is dark, brooding, heartfelt and mysterious and who loves (or loved if you put it that way) Lucretia. Yuffie is the exact opposite of who Vincent is, and you can screw the saying opposites attract because that kind of opposite is definately pushing it beyond too far and is just wrong!!! Why the hell would someone like Vincent like Yuffie? WHY? If anything he would probably want to keep away from her at all costs and continue his internal struggles, and help him if anyone should think a selfish person like Yuffie would have any kind of understanding or be selfless enough to care about him since neither of them spoke a word to each other during the game. Yuffie is just plain annoying, and if Vincent ever did see her he would hopefully shoot her with his Death Sentence gun and get the pain of looking at her hideous face out of his mind for more important matters.