The Joker is just not that great of a villain. I know you're thinking this is sacrilegious or some statement made by a grand contrarian, but you're wrong. I'm not crazy, misguided or on drugs. I promise you that. I just have read way too many bad Joker stories to keep lying to myself like everyone else does. Let me break it down a bit so you can see where I'm coming from.
1) He knows he's the bad guy.
Why does that matter? Because the guys who have the most to lose have the most compelling stories. The Joker has nothing to lose because he's just doing whatever he wants to be a jerk. The Joker mostly seems to exist in order to torment and taunt Batman, out of some grand need to tug on his pigtails, like some 5 year old that can't understand why he has a crush on his kindergarten classmate. He's not misguided, thinking that what he wants to do is the right thing. He's not trying to fix some other ill in his life and somehow screwing over Batman in the process. He's just constantly being a pain in Batman's ass because that's all he seems to be able to know how to do.
The Joker would be way more interesting if he was either trying to elicit a pained laugh out of the world or just had some small reason to poison the Gotham reservoir, other than just to do it. His sole purpose of existence seems to just be to torment Batman enough to annoy him. At least if he was Andy Kaufman, we'd be able to join in on the laughter and understand that he was doing it just to make himself laugh. The Joker rarely gets to the punchline before he gets his line punched out. The best villains usually don't know they are the bad guy. If the Joker saw himself as a hero, in some way, that could be really interesting. As it is, he just knows that he's a bit of an asshole.
2) His origin is ambiguous.
The 1989 Batman movie had this horrible idea of making The Joker tied to the origin of Batman. They revealed his name to be Jack Napier and that he was the guy who killed Bruce Wayne's parents. Why the hell did they do that? Because The Joker doesn't have a real origin. Yes, they've covered some possible ground in The Killing Joke and The Man Who Laughs, which are, in my opinion, the two best Joker stories ever told. Those two stories, however, are always flirting with being or not being canonical.
Now, I'm not saying that a good character has to have a concrete origin, however, if you're going to be vague about where a character comes from, it might be best to be way more vague and have less options for origins. They flirted with this idea in The Dark Knight film, which to me, was the best thing about that version of the joker. You had no idea where he came from and that made him extra dangerous, since you had no idea what his deal was. It made him interesting to find out, except in the film I felt like they never really came to a good conclusion about who he was or what the hell he was doing in Gotham, besides robbing banks and blowing shit up.
3) All he's really got is his smile.
You've seen him with trick guns, acid shooting flowers and smiling fish. He also has his Joker toxin that makes people have creepy smiles before they die. Those things don't really make him The Joker, those are just tools he uses to mess with Gotham and turn into a living hell for Batman and the public. Outside of that, a purple wardrobe and sometimes a hat and cane, what really makes The Joker? The smile, white skin and green hair are his most defining features, but it seems like writers cant agree on his personality. Does he even really have one? He is completely inconsistent with how he does things. You could maybe chalk that up to his insanity or being a maniac of pure chaos, but for me it always just felt like they could never come up with any defining characteristics for what makes The Joker who he is, outside of his appearance. Some writers make him hilarious. I love that Joker! Others make him speak as if he's The Riddler. I don't like that Joker, we have The Riddler for that!
Why is it so hard to make the character have any consistency, unless he's being handled by Paul Dini and Bruce Timm? The average person's favorite version of The Joker is from the Batman Animated Series, and that's with very obvious reasons: despite being chaotic, he was a consistent character. In the comics you never know what the hell you're going to get, but you're seldom going to get the charming and hilariously evil Joker that Mark Hamill voiced so perfectly. In the recent Death of a Family storyline, the Joker's smile was just there as window dressing. He never really seemed to enjoy what he was doing, after the grand reveal that he basically did no harm to anyone he kidnapped. He trolled Batman and then his dialog gave no hint to just how elated he should be for pulling the wool over Bruce's eyes. No jokes, no one-liners, no humor at all. Not much of a Joker there, now, is there?
4) How the hell does he keep hired help?
Dr. Doom has doombots. They are not humans and they don't need to think on their own. They are controlled directly by Doom and will do his bidding, sometimes taking his place and fooling the Fantastic Four or whoever else he's fighting at the time. The Joker, on the other hand, has hired goons that do his bidding. They have no real loyalty to a cause, since The Joker doesn't have one. They don't have a reason to rescue him if he gets caught, because he would never do that for them. He'll even kill them if they fail him or if he just feels like it. Who are these idiots that keep signing up for him? The Penguin at leas has the promise of wealth or maybe even health insurance, since he's well paid. The Joker rarely seems to be behind a heist or anything that is going to promise a decent payout.
Are these guys maybe forced to do it by the Joker holding someone hostage or dangling a family member over a pit of piranhas? That would be an awesome explanation for why they keep helping him, but it's just what I came up with, nothing that has ever been discussed. So, really, how the hell does he have guys working for him? He treats them like absolutely crap! I have no reason to ever think he could manage a crew like he does in the comics, which just frustrates me even more when I see him boss any of them around.
5) He's really not that smart.
Yeah, this is going to be the most controversial reason why I think he's not that great of a villain. I can hear it already. The Joker, by his very nature, is kind of an idiot. Yes, he's came up with some intelligent and elaborate schemes to fool and mess with Batman, but almost none of them have actually had any lasting impact, unless they were brutal and violent. None of his brainy schemes have ever really come to fruition or done anything that rocked Batman's world. The death of Jason Todd and the paralyzing of Barbara Gordon were both acts of brutality that were major game changers, but neither was a well-planned scheme or any kind of intelligent act. How about the fact that Gotham is so damn filled with criminals, crime bosses and kingpins, yet he still decides to roost there because he is obsessed with Batman. That's kind of dumb. Is his self-worth and ambition as a criminal so low?
It's like falling in love with a woman that has no interest in you, but you stay in your hometown and keep taking jobs that are low-paying and beneath you just so you can be around her. What the hell were you thinking? She doesn't love you! In the Dark Knight they at least had him take out some crime bosses and usurp the group by putting fear into them, but in the comics he often works along with all of the rest of the 17 billion Batman enemies and crime bosses. His self-esteem is lacking, his work ethic is poor and his intelligence is disappointing. Hell, that makes Batman look even worse, when you think about it. His greatest enemy is a bit of a failure unto himself, so what does that say about Batman?
Oh boy, you guys are going to really light up about that last sentence, aren't you?