It's a combination of things really. For instance, I am and always have been a huge Batman fan, but the two film versions were totally different for me.
I like Michael Keaton, but could have thought of at least 10 other guys I thought were better suited to play the part at the time. In the end, he did a credible job, as I thought Kilmer did as well. Clooney probably looked the part the best, but played it the worst. Those Burton type outlandish sets and backdrops didn't work for me, but I went to see the first one at the movies, simply because it was Batman. The rest I waited for DVD because I wasn't impressed.
The Batman Begins version was best for me because firstly, it stayed closer to the true mythology, with some variance to the beginning, but much more credible to the comics than the Burton version. Secondly, it was much more true to life, i.e., being possible that in could happen that way without being totally out of the realm of possibility. Thirdly, it was much more dark and dramatic which is the real Batman to me, and not some campy Adam West type junk.
That's just an example of what I'm talking about. Some variances are alright as long as they bring it to a plausable reason. For instance the genetic spider vs. the radioactive spider in Spiderman made more sense in this day and age. I wasn't thrilled about the change at first, but it made it more plausable.
The original Superman films took some liberties from the comics, but it seems that those became so popular that those changes are now the broadly embraced myth. And personally those films, along with the vibrant personality of Christopher Reeve, made me a huge fan of Superman.
Also, I wasn't too thrilled about the uniforms of the X Men being changed, but it makes more sense for them to be standard with variance for the individual X man because if it were real, it would likely be that way, and then I realized, the original X Men had standard uniforms anyway.
But in each one, the story wasn't so outlandish or cartoonized that one couldn't sit and watch the film and not think that something like that could happen without it being totally out of the realm of possibility. That makes a comic film for me and apparently other fans as well, because it seems that the movies that have been handled that way have done the best financially.