Uh, I was merely comparing my own reactions to the two castings. But now that you're speaking of the analogy between Heath and Ben, I still think you're off the mark.
In terms of Clooney, he didn't have the career behind him in 1997 that you seem to be referencing. In fact, he was mostly known for his role on ER, not for a long film career that spanned 15 years. Also, he was the second actor to play Batman in a 2 year period and was stepping into a "franchise" that was already 3 films deep. Not really the same as reimagining the role in a new series of films as Heath did and Ben is about to do.
As far as Heath's casting, at the time, he was mostly known for starring roles in 10 Things I Hate About You, A Knight's Tale, and yes, Brokeback. If anything, Brokeback Mountain is the only one of those movies in which he really displayed any acting chops. Sure, there might have been some ignorant fans who didn't want a "gay guy" as the Joker, but that was hardly the reason for his backlash. The backlash came from Heath being the least Joker-ish person who could have been cast. He had never, ever been mentioned as someone fans wanted to be cast and hardly anyone could picture him as the Joker, which is a similar reaction many fans have had towards Ben. Despite his good looks, no one was clamorong for him to be cast as Batman, and after he was cast, we saw there was a ton of people who didn't think he could pull it off.
Either way, despite whether they had similar careers before they were cast in these big roles, Heath and Ben share this similarity -- no one really wanted them to be cast in these roles at all before they were cast, and many people were confused, shocked, and disappointed by the initial announcements. Like Heath, Ben has the opportunity to surprise and impress a LOT of people, or he could fall flat. I certainly know which way you're predicting it will go...