I only know the Marvel side, but there are a lot of other major costume changes I can think of besides those already mentioned:
Hawkeye has been running around in the Black Ninja Ronin outfit for several years now. He’s just now going back to the purple archer look with Heroic Age. I liked both, but the purple suit wins out for me and I like him best as an archer.
Mockingbird (Hawk’s wife) had a black/white leotard with long sleeves versus more of a catsuit and goggles look now that she’s back from the dead. Hate the new look and wish they’d find a way to bring the old look back.
Wonderman changed his outfit a couple of times. He was introduced with green and red spandex and had his whole head covered with red goggles. The next rendition was just red sunglasses and a red overcoat. The later incarnation and one that has stuck the longest is black spandex with a red belt and a big red W on the front. I like that one best.
Ms. Marvel initially had Captian Marvel’s red and blue look. Incredibly lame outfit IMHO even with Moonstone bringing it back in recent years. The newer look is a black one-piece w/ long black gloves and thigh high boots. Yowza. Definitely a much better look for her.
Ant-Man (Scott Lang) changed from Pym’s original Red/Blue/Black design to a very different modernized version, though the character is totally different, so not really comparable. For the record, I do like the older suit better even though it’s more campy and less bug-like.
Thor has a very new look since coming back to life a few years ago. He’s had other minor variations in the past, but the new look is a little more streamlined I think. I like ‘em both, but took a while for the new costume to grow on me.
The Vision went from Green/Yellow to all white for a while. Pretty lame change and it didn’t last long (thankfully).
The “Iron Spidey” suit that Stark developed for Parker is actually still in use by another hero, a clone of MVP. Not sure what he is doing now that the initiative is over. I stilllike the Red/Blue or even the all black suit over these.
The Hulk went from dumb and green to smart and gray for several years. That was a great storyline, but more driven by the prospect of a smart hulk IMO than the gray color/outfit change. Green is still better.
Namor went from his trusty speedo to a much more covered up spandex look now that he’s hanging with the X-Men. This change is the most like WW’s change I guess in that he’s covering up a lot more. I was never really into the Sub-Mariner either way.
Iron Man has obviously had numerous changes over the year. The War Machine look spawned a new hero in Rhodey. The other one that stands out is the Silver Centurian outfit Start used to take down Stane and rebuild his life. I always liked that look. Changing his look is kind of his gig though and the red & gold looks are more iconic.
Hank Pym has had many different looks as well, but he usually changes his character with each. Goliath/Giant Man were the two most similar, but he also had very different outfits as Ant-Man, Yellowjacket, and now the Wasp.
The X-Men go through costume changes all the time and I consider their outfits a little less central to the characters. Storm, Rogue, Angel and others have all had some pretty wild swings in design. Wolverine’s brown costume stands out as a favorite for me. The Fantastic Four have minor changes to their outfits every few years as well, but they’re mostly all similar to the originals.
There are plenty more examples out there, but the really core characters seem to always return to more original looks (Superman had long hair for a while there, Batman had his Knightfall armor, etc.). Like everyone else, I think the new Wonder Woman outfit looks pretty bad – I don’t think it will last long at all.