After my post last night, I woke up to read Dave's suggestion for one-click subscribe. It would work for web-based aggregators, I suppose, but I don't like the centralization of it all. Couldn't one write a Firefox plugin to set preferred aggregator, and have RSS links automatically shunted to your aggregator? (I moved this talk to the updates at the bottom).
In any case, Dave's mention of all the brands and information providers immediately made me think: newspaper!
So, these portal sites are likely to have aggregation functions. Editors who pick out the best stories, re-categorize them, some original content creation, and of course advertising. Just like a newspaper.
Oh yes, one more parallel: aggregators take news feeds, equivalent to the custom wire services that newspapers use, and exposes them as "regular" web pages, which is what the mass market reads.
Update: Jeremy (and his comments) pretty much covers the options. For desktop, get a MIME-type registered. For online, depend on something like Firefox to hold your preference.
Just thought of this: a "my preferred aggregator" site -- click on the link, it generates the proper redirect to get the feed added to your online aggregator of choice.
It doesn't quite work for "personal" type news aggregation (i.e. desktop based, but that's where MIME-type would work), but again, the mass market is not likely going to do this -- they are more likely to customize info in the context of a more portal-like site.