What were you downloading? Some links to the pages where the problem is happening would help people to try to help you.
Or by download you mean to load a website? If so, http://www.tvguide.com/ loads fine here with no crashes; the same for Facebook.
Disable all extensions if you are using any and check if the problem still happens. Also empty the cache.
Well, Developer Stream is not only for the ones that want to test new features but also for the ones that know how to enable those features.
I may agree that Opera should make flags more visible to the public but i'm not sure if they should be enabled by default. Not of them are ready for it and not all of them would please everyone.
Please report bugs through the Bug report wizard. Do feel free to discuss them and collect more information in the forums, as well of course. You can send us additional information to DNAfirstname.lastname@example.org.