There's much of reddit that is still interesting and useful, and there's plenty of good discussion to be had - it just has a much worse signal to noise ratio since its exploded in popularity. If you're willing to unsubscribe from some of the bigger subreddits and take the time to "roll-your-own" so to speak, there is a lot of value to be had.
Right, there's really nothing fundamentally different about HN and reddit other than the audience. At their core they are both vote-based commenting communities - and the generally high quality of comments makes me wish more sites (like Gawker, Engadget, etc) would use the same mechanism. Yes there is a lot of herd behavior but it's leagues better than the "First post!" cruft you see everywhere else.
Well, there are subreddits for any particular political persuasion you may have - most of them have better discussion than the general /r/politics. There's /r/programming, but its a little light on content sometimes so I prefer:
/r/systems
/r/machinelearning
/r/netsec
/r/compscipapers
/r/coding
/r/compsci
/r/linux
/r/opensource
/r/csbooks
for the hacker part of me. There are also all sorts of other ones for any interests you might have - hard sciences, soft sciences, food, photography, sports, fantasy sports anything you'd like. Heres a spreadsheet with some other subreddits - its not nearly comprehensive, but it gives you an idea of what's out there.
TrueReddit is pretty awesome: http://www.reddit.com/r/TrueReddit/ . It's explicitly for "...thought-provoking articles, reddiquette, reading before voting and the hope to generate intelligent discussion on the topics."
In addition to what others have said, I'll add r/depthhub. It's similar to r/bestof, but it serves to collect all of the interesting and deep discussions that happen on Reddit. r/theagora is also great if you have a philosophical bent. It's an attempt to recreate the dialectic of the Socratics. Emphasis is placed on focussed rhetoric and honest argument, and to avoid logical fallacies.
