Now we're back to superhero comics which are in what is called The Modern Age of Comic Books, a synthesis of campy Silver Age classics, Darker and Edgier (where it actually works for the concept), and some Bronze Age-style commentary all trying to live in the "Real World" most of the time. Sales are dismal compared to the Dark Age (with brief surges in popularity following movie releases or Crisis Crossover storylines) and earlier, but trade paperbacks, movie options, and digital distribution help the comics companies to make more money with a less popular product.
Letters pages in comic books. While not completely dead (the odd comic book still has them from time to time) the rise of message boards, Twitter and Facebook has pretty much rendered them superfluous - why bother writing to the writer directly when you can just comment on his Twitter account? 2000 AD is a notable exception, where it's something of a Mythology Gag.
Supplemental articles and essays. Comic books often were like magazines. The writers often included inside the comics (often in the back pages) editorials, articles, short stories, and prose pieces that gave a look inside not only their creative processes, but also the work that goes into creating their comics. For the most part, this was discontinued quite some time ago to cut back on costs and leave more room for advertisements.
These changes are also related to changes in the law and buying habits of the readers. It used to be that having a page or two of text in a comic (a letters page counted) allowed the publisher to mail subscriptions at a cheaper postal rate. That law is different now, and most comics are no longer purchased through subscriptions anyway, so the letters page became surplus to requirements.
Comics based on real-life celebrities (invariably comedians) were enormously successful in The Forties and The Fifties. The Adventures of Bob Hope, published by DC Comics (!), ran for nearly two decades. Eventually they were finished off by the revival in popularity of superheroes.
Thought bubbles went from something that appeared in almost every comic to something that appeared almost nowhere. Their biggest extinction factor was probably The Dark Knight Returns and Watchmen, two books that completely eschewed thought bubbles in favor of either narrative captions or no internal monologue at all. The general consensus was that this looked better, served the same purpose, and took up less space, and the thought bubble has become almost unheard of since.
Todd McFarlane has fallen to this level in some circles, with people criticizing him for various reasons, especially his role in ushering in what some people view as the style-over-substance "Image Age of Comics". As a toymaker he's not so bad, though.
The Comics Code Authority was established immediately after Dr. Frederick Wertham's book Seduction of the Innocent, which painted comics with a broad brush as contributing to deviant behavior and juvenile delinquency. The CCA was a committee that forced comic book companies to police themselves. Basically, this meant no profanity; no drug or alcohol use; no nudity or sex; no blood, gore or graphic violence; no pejorative depiction of authority, government, or law enforcement, and a host of other restrictions. For decades, most mainstream comics carried the approval stamp. This necessitated the need for publishers to establish separate lines such as DC's Vertigo so that more mature themes were allowed. In 2011, Archie Comics, the last publisher to still carry the approval stamp, dropped it from all of their books. The parent organization of the CCA, the Comics and Magazine Association of America, is now defunct and the intellectual property rights to the CCA approval seal were acquired by the Comic Book Legal Defense Fund.
The original Clark Kent version of Superboy is now generally perceived as the character that launched the Legion of Super-Heroes but he was incredibly popular in The Fifties and early Sixties when he was easily the second most popular character in DC Comics and far ahead of Batman and The Flash in terms of sales. A combination of overexposure and changing tastes meant that by the early Eighties, Superboy was very much sidelined as a character and the Reboot after Crisis on Infinite Earths removed him from continuity altogether.
Dreamwave's Transformers Generation One comics went though this hard. When they were first announced, they had 'superstar manga-like artist' Pat Lee doing both all the promotions and a whole lot of art. The cast was straight from the old cartoon, coming during a period of 80s revival that ate those characters up like popcorn. It was advertised as a superb comeback, and sure enough, Dreamwave's entire original miniseries cracked the top ten in sales charts, with most issues even making it to #1. The success spread to the Transformers Armada comic as well, making it one of the only non-G1 comics to achieve mainstream success. They followed up with Transformers: The War Within, probably the most influential series to ever have Optimus Prime on the cover. Packaging art and merchandise of the time switched to a Dreamwave-esque style, and many of the designs of the various series (particularly War Within) would be incorporated into the following Transformers Cybertron.
But even at the height of its popularity, Dreamwave had its detractors. Complaints about fanfic-y plots and a fetish for the original 1984-85 cast abounded, and the general feel of the line reeked of faux-maturity. The artwork soon became one of the biggest complaining points, with "puffyformers" and "Dull Surprise" becoming common fandom terms to describe Pat Lee's art style (which all other artists on the payroll were forced to use). These complaints intensified with rumors that Pat Lee himself wasn't just a bad artist; he was a complete scumbag who refused to pay his employees, took credit from much better artists, and siphoned company funds into buying himself a Porsche. When Dreamwave went bankrupt from a mixture of flagging sales and Lee's embezzling, the public opinion of Dreamwave as a company flipped completely into "hate." Since then, aside from War Within, Dreamwave's books have pretty much vanished from the eyes of both the fandom and Hasbro (outside of the occasional Sunstorm toy), and many of the writers and artists who got their start there seem to regard it as an Old Shame. Even stylistically resembling Dreamwave books (using mostly the '84/85 cast, quotingTransformers: The Movie, killing off Puny HumansorGoBots) is enough to get alarm bells going in some circles.
The fall of Wildstorm is somewhat legendary in comics. It was a success story at first, being owned by Jim Lee at the height of his popularity and receiving attention from creators like Warren Ellis and even Alan Moore. Later on in 1999, it would be bought by DC, which caused no small amount of unease, but the launch of The Authority seemed to make the future certain for Wildstorm, becoming one of the iconic comics of the early aughts. But, as time went on, it became pretty clear that nobody at DC knew what to do with Wildstorm, least of all company president Paul Levitz, who regularly screwed with books and storylines. There's a clear trend of creators, often with long histories at DC and Vertigo, going to work at Wildstorm and then quitting the company forever - Alan Moore, Mark Millar, and Garth Ennis all saw some of their last DC stories come from Wildstorm. This also resulted in plenty of story lines being prematurely cut off or being left in the hands of people who couldn't care less about the comic, leading to massive Continuity Snarls. Ennis was particularly well-publicized; The Boys was one of Wildstorm's only decently-selling comics at the time, but Levitz refused to publish it, so Ennis took it to an independent publisher and bet that the relaunched comic would outlast Wildstorm itself. He turned out to be correct, as Wildstorm was dissolved in 2010 due to miserable sales.note The Boys ended in 2012. DC attempted to move the few properties that still had some name recognition to the New 52 relaunch, but all of them have been cancelled since then, leaving the universe's future looking increasingly grim.