Okay, just drank two cups of coffee (even though it's late) and I am ready for an in-depth analysis on Solas hate. Please excuse spelling and grammatical issues, I have the caffeine jitters!
I too find it difficult how anyone could say they "hate" Solas, even after taking a step back from my Solas obsession it's hard to discern what about Solas could cause something as passionate as "hate" for him in the fandom. As Elie Wiesel says “The opposite of love is not hate, it's indifference....". It is one thing to say you don't care for Solas or that you dislike him, but to say you hate him means he, a fictional character, really got under your skin and caused an emotional connection, even if it was a negative one. For example, I wouldn't say I hate any of the various Dragon Age companions, but I extremely dislike Anders (although I end up sparing him more often than not, but that is for another discussion). I could go on an essay long rant about how much I dislike Anders and why he is a horrible person, but at the same time, many people would jump to his defense and proclaim their love for his character. This, in my eyes, is a sign of a well-written character, someone who stirs various strong emotional reactions from the audience.
As we all know Solas is well loved by a very vocal fanbase, the fact that he also generates hate means he is a layered and controversial character, which, in my opinion, are the best sorts of characters written. For many fans of Solas he represents a wise, yet misguided lost soul. Someone whose constant efforts to fix the world only end up in sorrow and social detachment for him. For us, his is a tragic tale, made all the more heart-wrenching by the possibility of a romance that, as we know, will only end up being another regret added to Solas's mounting list. We see the best in Solas, and in some cases our adoration for him leaves us blind to his many flaws. For the players who hate Solas his flaws are magnified and his more charitable traits either ignored or not recognized. I imagine they see a character who, quite literally, lives so far in the past he that he doesn't know how to accept the world as it is, and instead persists to have seen a better world. He tends to stereotype people by their species and heritage and is genuinely perplexed when they deviate from these categories. I admit his expectation for Sera, Varric, and even your Inquisitor to be something they're not is a source of frustration for me. Fans of Solas may excuse this behavior, but not all players will over look this form of unintentional racism. Even more grating for many players is Solas's endless discussion of the Fade and ancient times and how he is far more knowledgeable about it and it's inhabitants. Now, Solas is more knowledgeable about the Fade and ancient times, but it isn't until the end of the game that it is made clear to us just how far his knowledge of these matters exceeds our own.
The realization that Solas is the Dread Wolf, the elven god of "trickery" came as a shock to many players, the fact that he also inadvertently caused the Breach only added to the shock. Solas, or Fen'Harel, has innocent blood on his hands, more than we probably know, and for many, this in itself is unforgivable. However, I'd assume the greatest sin of Solas's is his deception. No one likes to be deceived or to have their trust betrayed, and Solas did just this. Every moment spent with Solas, and every word from him needs to be re-examined, because when trust is betrayed so horrendously it calls all motives into question. Was Solas just using your Inquisitor? Were they ever truly friends? Is Solas truly sorry for the pain that he caused, or was he just remorseful for the fact that his plan failed? So, why would someone hate Solas, but forgive Blackwall or Anders? Well, not only is Solas's case of false identity Blackwall's x10, he also seems to be willing to sacrifice close friends for his vision of the future. All these things, coupled with many shallower reasonings (such as his insistence that spirits are people, his pushing Sera to be something she is not, and the fact he takes your equipment and leaves the party without saying goodbye) is enough to cause people to have extreme negative feelings towards Solas.
At the end of the day, I find any opinion that can be supported with logic and reasoning to be a valid one, and that there are very valid reasons for hating Solas. Solas sparks controversy, yet is still adored and respected by many fans, the fact that he is hated by some people is not something fellow Solas fans should be upset about, instead we should celebrate the fact that he is such an interesting and significant character. We can be sure, whether positively or negatively, Solas will be a Dragon Age character that will be remembered far into the future for his role in the games and the turmoil he caused fans.