You can already do this yourself with yahoo pipes & google reader. I'm not really seeing the value - especially for programmer types! Now, for less technical folks it might be more valuable. However, they may not realize that they need or want such filtering and aggregation.
The value is that you have a feed of relevant posts and can easily discover new interesting people.
Seems similar to coder.io - you subscribe to hashtags like #mongodb, #ruby, etc and it spits out a list of stories
Tagmask provides more flexible filtering and allows users to submit different types of posts (Link, Note, Code snippet etc)
I can see this being really cool. I'm interested to see where it goes.