These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Harry Potter and the Nightmares of Futures Past
Alternative Character Interpretation: Harry, for obvious reasons. He comes across as having more of an American action hero feel to him; he keeps a gun because it might come in useful, he drawls, he's quick to throw witty and sarcastic remarks at authority figures, and he draws a lot of inspiration from American military procedures. It does make sense as older!Harry did spend a lot of time with the Americans following the deaths of his friends but it can be jarring when comparing him to the very British school boy he was the first time around.
Mr. Granger gets the biggest change of anyone, as he is turned from someone who is implied to be a bit timid from canon sources into a thoroughly stubborn, opinionated, highly demanding overprotective father who reacts negatively to the idea of boys interested in his daughter and was actually prepared to pull Hermione out of Hogwarts with no thoughts as to how she felt about it. He even blamed Harry for the racism present in the magical world. This is especially jarring considering Mr. Granger is supposed to be a dentist, yet he's the most hard-assed dentist one could ever hope to meet.
Base Breaker: For the same reasons as Team 8. It's partly due to both fics' overbearing popularity, but a number of readers simply dislike the way in which many events and characters are depicted overall (and this isn't just limited to Shipping). You're either going to love this fic, hate it, or respect the auhorial style while disapproving of the plot. No other grounds seem to exist.
Epic Fail: Harry threatening to start beating on Hermione if she hurts Ron Weasley, never mind that her future counterpart was loyal and faithful and loved him till the very end, even pushing Ron out of the way of a fatal curse at the cost of her own life. Way to fail, Harry.
Hype Backlash: The level of attention and praise it gets in forums as a Fanfic Mastership can colour the expectation of future readers to impossible levels.
Shipping: The story follows the canon ships of Harry/Ginny and Ron/Hermione, but it also tosses in a new one: Neville/Luna. (As if you didn't see that coming.)