My first comment is that you have to think about content management systems differently than you do other kinds of computing platforms. CMSes basically use documents, spaces and rules to run things.
If you have a rule that adds an aspect, and it is triggered by, say, a document appearing in a space, you could then cut-and-paste the documents you want classified into that space. Once they have been augmented with the categorizable aspect, you can cut-and-paste them back to where you want them.
Edit:
Thinking more on what I wrote, I can see that the suggestion that I made is usable, but very clumsy. Very, very clumsy. Alfresco Web Explorer definitely needs to have an Apply All or Apply to Selected operator that allows you to perform any of the operations that are selectable; delete, execute a script, apply an aspect, etc.
