Mac OS X 10.6, a.k.a. Snow Leopard, no longer uses an old Mac method of deciding which application to launch when you open a document. The upshot of it is that, by the old method, a document would usually be opened by the application that created it. With the new system, all documents of the same type will be opened by the same application regardless of which application created the document.
Also, the system will ignore the older way of marking the type of the file in favor of filename extensions. (Which are the “.html”, “.txt”, “.doc”, &c. at the end of a file’s name.)
There is still a way to tell the system to open a specific file with a specific application, but this can only be done “by hand” through the Finder. Other programs cannot safely do this.
“Metadata madness” by John Siracusa does a good job of explaining it all in more detail.
Personally, I avoid letting the system choose what app to open files with unless I am very sure of what it’s going to do. I can’t disagree with those who say that the functionality of creator codes ought to be restored, whether in the old form or a newer one. I won’t personally miss it, though.
I think something like Quicksilver is the future. Quicksilver is an application that allows you to quickly find documents and specify what you want to do with them. Even better, it learned from the user and was expandable. Google Quick Search Box seems to be the successor to Quicksilver, though it hasn’t caught up with all of Quicksilver’s features yet.
The biggest problem with the old Mac creator and type codes is that there was never a user-friendly way to manage them. Users could easily get into a situation in which they needed to set or modify the type code for a document, but the system didn’t give them a way to do it. There were utilities to do it, but they typically exposed the codes directly rather than providing the good user-interface you’d expect on the Mac.
UTIs are fine upgrade for type codes. Although, I’m ambivalent about UTI’s depending upon filename extensions. Filename extensions are an awful way to store file type metadata. Yet, I can’t argue with their practicality.
Examining the contents of a file to determine its type is often better than any type of metadata, though there are times when metadata is better. (Mac OS X actually comes with a Unix command-line utility that determines file type by contents.) I think a combination is probably the best choice.
Putting aside the issue that the system doesn’t use bundle identifiers in the same way, they are a fine upgrade for creator codes.