Top of the list then:
Good Writing.
This is the flip side to poor writing. Because when they get it right, it is simply wonderful. Good writing can make you ignore mediocre artwork. Good writing can make you remember a story and go back and read it again and again and again. Good writing becomes part of the lore and continuity of a particular character, and REALLY good writing can provide future writers with a wealth of opportunities. Good writing can create new and wonderful characters. It can restore the reputations of old or obscure characters, or redeem a villain and cast a hero in a whole new light. And there are some simply spectacular writers out there, producing comics for our enjoyment.
Good Art
While good writing can redeem artwork, sometimes really amazing artwork can lift up a so-so story to heights it couldn't otherwise aspire to. Again, art is subjective, I think more so than writing, but some artists are pretty universally recognized as being masters of their craft.
The Whole Idea of Sequential Art and Words
Simply the actual FORM of comics is rather amazing when you come to think of it. I'm an insatiable reader and have been since I was old enough to figure out the alphabet, but the idea of putting pictures in with the words is pretty damned brilliant. It's also pretty damned old, if you want to go back to the cave paintings, hieroglyphs, illuminated manuscripts and political cartoons. I think it was Boss Tweed, who lamented that people couldn't necessarily read, but they could sure understand those pictures by Thomas Nast.
We start with the funnies in the newspaper, graduate to comic books and even animated cartoons. It's been labelled childish of course, but there is an astonishing amount of sophistication in a great deal of sequential art.
Silver Age Madness.
There's nothing quite like the insanity of older comics, when they were still a fairly new medium, at least Super Hero comics. Some of those old Superman and Batman and Green Lanterns stories are INSANE! And that's part of the fun of course. Where else but comics can you have invisible women, people who can fly and wear their underwear on the outside, mythical gods come to life, magic rings, wirepoon guns, flying bathtubs and Spider Senses? Need a particular trick to defeat the villain? Come up with Freeze breath, or the ability to get gold out of sea water. It may never be used again, but on the other hand...it might just stick.
Super Hero Universes.
Marvel and Dc each have their own unique worlds, and after years and years of storytelling, they have their own quirks and characteristics. There are multiple universes, or at least dimensions, lots of friendly and not-so-friendly aliena, and truly bizarre laws of physics. Normally exposure to radiation will kill you, or at least make you deathly ill. Not in the Marvel Universe, where it will give you all kinds of really really cool powers. Which you will then use for good. And the ability to angst unendingly is looked upon as a handy talent to have.
Super Hero Locations
The idea of individual Universes rather flows into this idea. DC in particular has such wonderful locales. There is Opal City, Gotham City, Metropolis, Star City, Central City, Coast City...and on and on. And each city has their own particular super hero...their own mascot as it where. They can go and visit each other, but they are really pretty much associated with their own places. Batman is in Gotham, Superman in Metropolis, Green Lantern in Coast City and so forth. Marvel for the most part only has New York City, which is a bit of a shame, as you can't swing a dead cat without hitting either Spider-Man or Daredevil or Power Man. No wonder they fight each other all of the time, they are trying to protect their tiny bits of turf!
Then there are places like Oa, or Krypton, or Thanagar or Rann or Titan. And there are wonderful alien races to go with those alien places. Skrulls, Badoon, Kree, Qwardians, Korugarians, New Gods, Bolovax Vickians.
Aliens with four arms, aliens with no arms, or flippers, or fins or beaks or gills or androids...it's all just so rich and wonderful, an enormous toy box for us to dip our hands into and stay young.
Really Fabulous Characters.
The actual characters of course are my real love. I still believe that the Green Lantern Corps is simply one of the best ideas that DC ever had. I love the Justice League and the Justice Society and the Avengers and the Defenders and the Champions and the Legion and the Detroit League and the Great Lakes Avengers and the Inferior Five and the Fantastic Four. There aren't a whole lot of characters that I DON'T like, although there are a few of course. But that goes back to good writing, a good writer can make an otherwise mediocre or obscure character come to life. Just look at Catman. There is a hero or heroine for everybody's taste, with an astonishing array of powers and personalities.
Villains
There's nothing like a good villain. You just can't have a proper Super Hero without a good villain, which is probably a truism by now, but still...there you have it. Sometimes I like the villains BETTER than the heroes. There are a lot of crappy villains of course, but heck, even those can be a whole lot of fun in the proper hands. My personal favorite completely obscure villain was Carface, who showed up in about three panels in a Birds of Prey issue, and was never seen again. But...but he had little teensy windshield wipers for eyebrows! My heart just melted.
There are some simply grand villains. Darkseid, Sinestro, Doctor Doom, Lex Luthor, the Joker, the Rogues, Thanos, Brainiac, Mr. Mind...yeah, I love Mr. Mind. There are villains that aren't really so bad deep down, like Catwoman and the Riddler, and the Secret Six, and even the Kingpin wsa just being a good business man half of the time.
Violence.
Man, there's nothing like Comic Book violence. I can't go out and kick someone in the face, but there is a certain visceral thrill to watching Batman do it. There is the insane joy I get from seeing Hal Jordan get whacked in the head for the millionth time, or having Spider-Man web up some hapless criminal and leave him hanging from a lamp post. New York City gets blown up every other Tuesday, and life just goes on. Sometimes it's just so...so satisfying.
Sound Effects.
Let's face it, sound effects are just fun. They are also something that really can't be done in any other medium except comics...although the old Batman television show certainly tried. But a good thwaack, or Klunch or Kraakkaadoooooooom are just sensational. The Incredible Hercules book has had some of the best sound effects that I've ever read and they contribute so much to the general tone of the story.
Weird Science.
There is regular Science and then there is Comic Book Science, and let's all just face, it, Comic Book Science is a heck of a lot more interesting. Origins are happening all over the place, just by the simple mixing together of odd chemicals, rampaging gamma rays, genes run amock and certain magical properties just lying around. Weird Science allows us to have intelligent Smallpox viruses and sentient planets. Spaceships that can span galaxies in a matter of hours. Flight rings and force fields, and repulsor rays and dissolving webs. If you want to be a villain, and need to have some sort of schtick, there is a handy mad scientist who is more than happy to service you. There are a lot of mad scientists out there anyway, with their malfunctioning cricketrons and emotional robots and time machines.
Green Lantern Butts
Come on, you knew I was going to mention this one. Yes, I'm twisted, and I just don't care. There is nothing quite like a really well-drawn Green Lantern behind.
I just love my comics. And I love them a whole lot more than I hate them. Which is the way that it is supposed to be.
