This new flavor could end up being the sweet spot between the Delicious we've come to know and the interesting but relatively niche TrailMeme (http://trailmeme.com/). I can see some interesting hacker focused stacks emerge that aggregate quality links around, say, getting started with a new technology or maybe fund raising.

The challenge with "stacks" though, as with any list curation web app, is 1) maintaining the quality of content and 2) surfacing higher quality collections as you scale and become more and more inclusive. And these are hard problems to tackle without having humans sift through thousands of stacks to pick the diamonds in the rough (Visit http://www.imdb.com/lists/ to see what I mean).