Some things surprise us like John Carter, The Lone Ranger, and Green Lantern. I really didn't think any one of those could be a failure and the list could be much longer. I'm not sure what the answer is for how to make a successful D&D movie but it seems there are as many negatives as positives for each setting and known character in the D&D stable, not the least of which if you do one many folks are upset you skipped their favorite. It's a divided audience before you even put pen to paper, and that's your 'base' audience, the ones you would hope were already in your corner.
John Carter: No surprise, really. It's not that well known outside geekdom anymore. Good movie... not even all that unfaithful... except for the clothing issues (namely, that there was any).
Lone Ranger: Haven't seen it. But when Depp plays Tanto, I begin to wonder what drugs the casting director is on. Plus, the previews were horrible. Lots of flop potential. So, no real surprise there.
Green Lantern: It's DC & not Bats and & not Supes... it's doomed. DC hasn't had a successful movie that wasn't Bats or Supes in decades.
The real surprise is that DC has succeeded on TV where they keep failing on the big screen. Arrow and Flash. (Not sure about Gotham... I know that I gave it a shot, and didn't care for Gotham.)
And the thing is, the Bat-fans of the movies don't seem to be the same group of nerds as the Bat-Fans of the comics. A large overlap, but a lot more like me: I watched the 60's show as a kid, in reruns, then the Keaton films, then the Nolan films... But I don't think I've ever read a Batman comic. My friends who read the comics whinge on about the movies for the most part, and cringe when confronted with Adam West.... Likewise, I've read maybe ONE superman comic. I've seen the movies. Including some of thethe old B&W TV shows. But both sides like the toys and games...
D&D movies, if done well, don't need to be nearly as iconic, but they need to be done well. I'm not certain Courtney Solomon can do anything well. We need to see stuff that can be reasonably replicated by the mechanics. In fact, being too faithful can be as bad as not faithful enough.