Other than the things mentioned already (color, iOS 7, plastic, etc) my main dislike of the 5c stems quite a bit from the dislike of the 4 and the 4s and what I see as a betrayal by Apple.
The iPhone 5 was my first iPhone. I saved a portion of our tax refund for about eight months in order to get it. Metal, black, everything I wanted.
I hate the 4 and the 4s with a passion. I waited a year for Sprint to get the iPhone 5 because the first iPhone they get when they finally get the iPhone are these two models I don't like!
I expected Apple to shift the iPhone 5 down in price as a lesser model this year just like the did with the 4 when the 4s came out and just like they did with the 4s when the 5 came out.
I did NOT expect them to do this, wrap the metal 5 in a new plastic case, make it look fugly with uninteresting colors (and NO BLACK) and then upgrade the internals a bit! Nor did I expect Apple to market it as NEW!
And insult to injury, the iPhone 4s was given another year of life!!!
So what this tells me is that Apple values the 4s more than the iPhone 5, which was it's flagship for a year. I resent that because I see the iPhone 5 as the superior model to the 4 and the 4s. But Apple apparently thinks the iPhone 5 wasn't good enough to leave alone, but the inferior 4 and 4s was.
All of this is what I feel. The reality is that Apple had issues with the annodization of the iPhone 5, so turning it into the 5c and reducing the cost to build it by using inferior plastic, made sense to them. Despite the fact that I hate the 4 and the 4s, the world still seems to love these phones (I have no idea why) and Apple still makes money from them, so keeping the 4s around still makes Apple money.
Apple is a business and like any business, exists solely for the purpose of making it's stockholders money. So, my feelings are irrelevant to Apple's bottom line.
I will make my obligatory statement here that I don't look down on anyone who likes or wants the 5c (or the 4 and 4s). It's just that these phones are not for me. I like my iPhone 5. I did NOT get the 5s because Apple chose not to have one in black this year. Major dealbreaker for me.
However, as I have said in one other thread, when Apple finally kills off the 4 and the 4s - I will dance a jig on their graves!