Three main approaches:
1) The moral/ethical/etc. concerns. I might be different from a lot of HN people, and maybe similar to your friends, in that I'm pro the foreign military/intelligence spying, but I'm anti the normal-people and service-provider spying (US or domestic). Everyone has to make their own decision there, and I personally think this is the worst argument to make, since people will either fully agree or won't ever agree.
2) Practical/personal concerns. NSA is a closed community. If you work there, there's some sharing internally, but you can't participate in the outside community. Back when NSA was the only game in town for crypto, sure, but they're certainly not the only game in computing, development, or even in exploits. Ghettoes suck. It's not good for your professional development.
3) Negative-sum/emotional. Basically, NSA is preventing bad stuff from happening by doing bad things. Working with that for your whole career is kind of shitty and depressing, and you end up locked into that forever due to #1 and #2.
I've done DOD/USG work, and actually wanted to go ROTC and NSA in college, but ended up not doing so because of these reasons (and that under Clinton, and post-cold-war, NSA/USM/USG seemed to be moving toward irrelevancy; they were technically incompetent, too. I couldn't predict 9/11.)