I like Flareon personally, but I've never actually used it for the main reason that has been mentioned in pretty much every post- while it has that huge attack that could give it potential to be a great addition to your team, its strongest moves are Flame Charge and Fire Fang. While the first one gives a nice speed boost, they're both pretty weak. You've also got Facade but you'd have to carry a Toxic Orb and that can be quite inconvenient. So the main reason people would hate on it is that it's just not bringing out its potential- it could be a badass Pokemon if it carried moves such as Flare Blitz, but it doesn't so I guess we're stuck with this well-designed Pokemon that's poor for battling.
Add in the fact that it isn't exactly that bulky and with a relatively low speed, I guess that's why people would hate on it. And then there's the competitive battling side- Flareon would just not cope well at all, even in the depths of NU, since there are much better things out there in my opinion. Overall though I do really like Flareon's design, I'm just disappointed Gamefreak didn't bring out the potential in it. :[