But they are based on characters from a VISUAL MEDIUM. The way comic book characters look (in the comics AND in adaptation/translations) means EVERYTHING, whether people want to admit it or not.
		
		
	 
THIS RIGHT HERE, is what many do not get.  It is the reason MANY are upset with this adaptation....because though some changes will happen, the major changes to visual concepts of the comics do make a difference to the fans.
There are going to always be some changes, sometimes because the visual on the page does not translate well to the screen...but none of the changes we have seen thus far fall into that category, they seem to be change for the sake of change.
For me personally, nothing in these changes is all that major for me, except that Mara just does not have the look of the Sue I have seen over the years....ALTHOUGH, she does bare a resemblance to an artist or 2 from the UFF so I'm not so upset with that choice, I will give her a chance for sure.
As far as the others, it's not a major OMG  they look nothing like the comics, but I have to admit, the cast certainly don't jump off the pages of the comics.  AND....I want to see Doom in all of his glory, just to see how that comes off on the big screen.  I wasn't real thrilled about seeing a big purple guy named Galactus, but I was very curious how it was going to be done, and we got a cloud..... 
I think my main problem is that so many are so quick to jump to conclusions when it comes to motives behind why so many are not happy with the cast...when it boils down to exactly what you said, and really nothing more....and there is absolutely nothing wrong with that....