I think you're right, Nick, that there's a certain amount of secrecy in these lines but they really don't need it anymore. Especially with how much about a movie gets out online these days, there aren't a ton of secrets for people unless they really stay away from spoilers. But in that case, I don't think a toy line is going to blow anything.
There might also be another issue also, and that's the lead time on the production of the figures. They have to be in production while the movies are still filming and on some of these superhero films that have a ton of CGI work, Hasbro, Mattel and the other companies may just not know exactly what a character will look like until it's too late to get them into the first wave or two.
The original Elan Sleazbagano in the AOTC line had ears that Hasbro had to remove because Lucas, or someone, updated the character's look after Hasbro was already producing the fig. It happens, and it probably happens more on secondary characters than the main.
Still, it's really frustrating to see a line like Thor or Captain America launch and it's all the hero with no variety. Still, neither of those lines was as bad as Iron Man 2. That line is a model for what not to do, ever, with a toy line.