I spend a lot of time bouncing around from one site to another to another as I'm interested in the topics they have to offer. As stack exchange continues to grow, and as I continue to find more and more sites that interest me. Because I don't always have the time to go to each site to see what interesting things are on them all, I typically will go to my top 3 sites, and then one or two other sites each day.
I propose a portal, something like
my.stackexchange.com where you could add tags and have it crawl the other sites to generate the questions you're most likely to be interested in. It also could use some clustering algorithms to determine questions you're likely to enjoy based on their tags and the tags to other questions you've viewed, voted, answered, or otherwise interacted with.
I know this is similar to the RSS feed of your favorite tags question, but I think it is significantly different and of a bigger scope. The bottom line is StackExchange needs to be able to scale with the amount of time users have available to it.
I'm also open to suggestions and alternatives (and constructive criticism!) to this idea.