That really is the question, another frustration I have with my new iMac. The operating system is so accommodating, it happily does whatever you tell it to do. Or not. But no matter what, it never complains. Users may not like to see all the error messages you encounter on a PC, but at least with the error messages you know what's going on. The iMac doesn't return any error, ever, from what I can tell, it just silently ignores your command like an obstinate child.

For instance, when I tell iMovie to import a really big .AVI file, it appears to do so. It grays out files that it can't import as movies, and the AVI files aren't grayed out, seems like it should work. After selecting an AVI, it even tells me it's processing afterwards. Then I sit and wait, wondering just how long it takes to import. But it never does import the AVI video because iMovie can't import AVI files, which you can find out by research, but certainly not by the program telling you or returning any errors. 

Do you know, the index of the Mac book I bought doesn't even have "error" in the index! Perhaps this is why Mac-nuts rave about how great it is?