Wildcard support is a great step, however I'd like to propose going a step further. For smaller sites, it isn't as necessary to limit things by tags because you aren't getting overrun by the immense traffic like on SO. For StackExchange.com to truly be a place for aggregation, instead of just one more place to look at, it'd be great to be able to say show me everything from WebApps plus [sushi] on cooking plus [c#] on SO for instance.
Done. Just click the add entire site link at the top of the "Add Tags" section to toggle into adding a site mode.
Also, as a counterpoint to Jeff's argument, I believe that this does fit in well with the philosophy of the feature, which is show me all the stuff I'm interested in across a bunch of sites. Jeff's right that if you want to see every question, the right place to go is still that site, but maybe I want to aggregate several smaller sites and watch all of their new questions in one place. Now I can :)
I don't think this fits well with the philosophy of the feature. It's about questions with specific tags, not all questions.
If you want to see every question on a site -- go to that site!
Tag sets are really for the sites you don't live in, but you might like to visit there once in a while.