The reason the movie adaptation is 'so hated' is because it destroys the ideas of the graphic novel. This may seem a bit heavy-handed, but after reading the original a few times, it's pretty damn evident.
Most obviously, V's message. His message isn't that 'everyone wants freedom, blah blah blah, etc.' He set out to do two things: first and foremost, destabilize the government into a position of anarchy from which the people would establish a new government. V didn't need the support of the people. He wasn't staging some stupid protest. He set out to blow up the key centers of operation and leave the people with a blank slate, whether they liked it or not. And that reminds me, the order of explosions was entirely wrong - what about his monologue to Lady Liberty? What about blowing up the radio tower? They cut out the whole thing where he differentiates between anarchy and chaos (ironically, the scene where the guy shouts "Anarchy in the UK!" is exactly what V defines as not anarchy). Also, the focus on weapons...? They cut out pretty much the only part where he actually used them (bomb hand, anyone...?), and injected them into that prolonged, horribly tacky scene at the end. The V of the graphic novel is much, much more reserved, and hesitant to clue Evey in. He's more mysterious, to the point of giving Evey a riddle instead of instructions when he's dying. He doesn't spell everything out for her like he does in the movie. The movie, of course, focused on V being an emotional, justified hero out for revenge.
Evey. Evey was supposed to essentially be a product of this system that happened upon V. The attempt at prostitution was symbolic of this notion. Instead, she's shown as mature and rebellious from the beginning. No. She grows into this, and at the end, her taking up V's mask was one of the most powerful moments of the book. Of course, everyone else in the city put on a mask in the movie, but they couldn't put one on Evey. I mean, "Reports of my death have been... greatly exaggerated" is one of the best lines in the whole thing.
Finch. Finch was done fairly well, but they delved way too far into Larkhill. Most of that was left up to the reader in V for Vendetta, and the whole 'antidote, disease, epidemic, testing' thing was completely added in. They were concentration camps, nothing else. Also, Finch was supposed to kill V - he's almost like Rorschach in this way, he has to abide by his personal moral code (in this case, protecting the government), regardless of whether or not it's the best thing for society. He doesn't do it out of corruption, but rather out of duty, which is what makes it more powerful when he is set to become the next V after Evey - he would become the dutiful enforcer of an anarchy, as the society he sought to protect crumbled anyway (all for the better, of course).
Adam Susan/Sutler. Come on. What the hell? He was a decadent, creepy geek huddled over his supercomputer, not a lean Hitler wannabe. He wasn't killed by his own government, he was killed by the wife of a middle-tier government official who had died. The way V drove him crazy by hacking into the supercomputer was very poetic and eerie, as was the instance where he thinks 'Hey, maybe I should relate to the people' and then gets shot on his first attempt.
I'm not even going to delve into the numerous subplots that they completely left out, because that was bound to happen - it's a long graphic novel.
Still, changing the entire ending? Necessary? I don't think so. Having hundreds of Vs march up was just stupid, and it defied V's M.O. and the strength of the ending entirely. Yes, it may be hailed as a good movie, whoopie, it was fun, but it wasn't great, and it was a terrible adaptation. V's monologue at the beginning...? Really? He wasn't a fun, lovable, cuddly guy wearing a mask that just used lots of words starting with V.
Of course, the Wachowskis seemed set on making a box office hit, rather than any kind of decent adaptation.