A common thing I often see discussed in superhero comic circles is the Marvelization of DC. For example, Roy Thomas DC work is often seen as a Marvelization of Golden Age DC characters. Same for Denny O'Neil's work and Jim Shooter's early Legion work (All 3 have said in interviews that they were huge Marvel fans, despite getting their big breaks at DC comics, and specifically wanted to bring the Marvel style to DC). More examples are Englehart's Batman, Gerry Conway's DC work, Marv Wolfman's Teen Titans, Byrne's Man of Steel and Miller's Batman work.
I see this discussion come about often. Breevort even revisited this issue recently. But one thing I don't see discussed often is how Marvel over the years is just as guilty of DCificating their books as well.
John Byrne made a great point that DC used to have its house style and stable of writers and Marvel had its house style and stable of writers (really, just two or three) and for the most part there was little overlap and the house styles were very distinct. Then came into the industry a bunch of fans-turned-pros who grew up reading both companies and also jumped back and forth between each company as pros and eventually the house styles began to blur.
A big example is dilution of characters. DC was big on diluting characters (Batman, Batgirl, Bat-hound, Bat-mite, Superman, Supergirl, Superboy, Krypto, Streaky, Beppo, Comet, Bizarro). Marvel not only didn't like dilution, they openly mocked DC for it, for example in Not Brand Ecch! where they spoofed the Weisinger Superman.
Yet nowadays we have Spider-Man, Spider-Girl, Venom, Antivenom, Carnage. We have Hulk, Red Hulk, She-HUlk, Red She-Hulk, A-bomb, Savage She Hulk, Skaar Son of Hulk and who knows who else. Captain America and US Agent. Iron Man and War Machine and Iron Maiden. Wolverine, Daken and X-23. They dilute their properties just like DC traditionally did.
Stan Lee hated sidekicks and teen version of adult heroes. That's why he killed Bucky when he brought back Cap. There was only one teen hero, Spider-Man, and he was on equal par with adult heroes. Over the years though we have different versions of Marvel's Teen Titans: New Warriors, Young Avengers (which has teen versions of adult heroes), Runaways...the list goes on.
Another way Marvel has DCified over the years has been to gradually turn their characters into legacy characters, even if the changes didn't stick. Ben Reilly Spider-Man, various Captain Americas, Thor passing the mantle to Eric Masterson, Rhodey being Iron Man for a while.
What are other ways each company has become more like the other?