Opera actually has this nifty feature called "site preferences" which allow per-site customization of everything. I have flash (and java) turned off globally, and only turn it on on a case-by-case basis when a site (like youtube) really needs it.
Right-click into a page, Edit site perferences... turn on flash in "content".
Opera's "Block Content" feature does the converse - you click on stuff that you *don't* want to appear on certain sites.
Also, Camino (which is a Mac-conformant UI around Firefox's rendering engine) has a feature where flash plugins only play after you click on them.
Problem solved, pretty much!