Let's see. Originally, I didn't have any quarrels with Demise. I didn't like him. I didn't dislike him. After sitting down and shifting through my thoughts, however, I have had time to analyze Demise and have come to the realization that I don't like him. (Random Person dislikes something from SS! Big Surprise![/sarcasm]).
Design
Can you say "generic?" Big, buffed up, ultimate bad guy has been done way too many times in the era of villainy. Even a villain design that's been done too many times can still seem fresh in the mind, but Demise just wasn't. The only thing really interesting about him was his fiery hair. When I see him, plus the fact that I know his story, I can only see Nintendo trying to make someone who looked like Ganondorf, but wasn't Ganondorf. If you're going to make an original villain, please make him original and not a rip off of who we love to fight. (<-- That comment probably made some people mad. trollface.exe) And if you're going to make him Ganondorf, than make him Ganondorf. All in all, his designed seemed like a failed attempt to give fans the best of both worlds of having Ganondorf as well as an original boss. (Personally I blame everyone who complained about the G-man's appearance in TP)
Personality
I didn't hate his Personality, (I do agree with Koolsholts though. That part was poor design) but I have issues with it and it's because of something I said before. Demise was basically another Ganondorf and I'd much rather have Ganondorf than a rip off.
Battle
I did not hate the battle with Demise, but it left me rather disappointed. People say he was easy, but I actually had trouble defeating him and had to use several potions. This is because, however, his fighting style is pretty bland and I kept trying to change tactics when I should've been doing the same thing over and over. Automatic defenses, while containing the possibility of difficulty, prove to be generic and entertainment lacking once the pattern is figured out. I may have struggled with him trying to best him with my swordsmanship skills that I apparently don't have, but others could easily exploit the pattern, especially those good at shield bashing. This also takes away from the fun, because there's no more "figuring." A quarter of the fun with Zelda bosses is figuring out how to defeat them. Difficulty isn't the only thing though. Demise impressed me with shooting lighting from his sword and allowing me to do so as well, however, the fact that he only has two stages, both being really short and repetitive, takes away from this. If you were to count the part with Ghirahim as part of the final battle, then maybe you would have a better case, but I Personally didn't feel that to be true as the atmosphere of the game did not seem like it.
Most final bosses in Zelda proved to be either a great challenge for their time, and/or very creative in being defeated. I feel that while Demise didn't completely fail at these aspects, he could've done alot better.
Concept
And then there's this category. Oh boy, forgive me if I rage. I'm not gonna beat around the bush, I HATE what Demise stands for. An ultimate demon who wants to rule the Earth is fine. A bit generic, but fine. It's been done in Zelda before and I'm one of those people who doesn't complain that much about Zelda traditions. But the fact that Nintendo stuck the "He's essentially Ganondorf" excuse on him makes me want to rage. It's seriously a cop-out to try and please everybody, (and I really feel that it failed). If you want to give us a new villain, by all means, do it. If you want us to fight Ganondorf, THEN GIVE US GANONDORF! Don't make up a villain with no previous mention in any Zelda game and tell me he's the reason Ganon keeps coming back. Not only does this take away from the epicness that is Ganondorf (saying he's only returning because of Demise's will) and the story of the Zelda series but its a prime example of what I hate about SS. It tries to please everyone by making sacrifices in the wrong places. I feel like I've put more dedication into this game series than Nintendo did simply because of how non fluent SS is. I'm all for trying to please more people and all for trying new things. I liked Twilight Princess and the different things that came with it. But SS's changes made it so that it didn't feel like Zelda anymore but rather a game with "The Legend of Zelda" on the title. Hence why we got a villain who wasn't Ganondorf, but at the same time, still was Ganondorf.
All in all, Demise is not the worst villain in the world, but he certainly disappointed me. He's lacking in originality, creativity and his genericness (not a word) is in all the wrong places. Given what I experienced at the end of SS, I might have been happier thinking that Demise was a giant fish with appendages, marshmallow toes who could fly. Believe it or not, I actually liked the Imprissoned more than him.