1) Unsubscribe should be one click, no confirmation. In the case of accidentally unsubscribing, if it's really that valuable, the person will re-subscribe, and I guarantee it's easy to subscribe because that's already optimized, for obvious reasons.
2) GMail shouldn't give you the option to choose; if you mark something as spam, it should silently attempt to unsubscribe in the background, then keep track of which services don't actually support unsubscribe, and mark those as spam. Far too many people are vindictive and lazy and will intentionally mark things as spam either because they don't want to go through the process of unsubscribing, or they forgot they subscribed, or think they've been "wronged" by getting an email they opted in to.
For the record, I too despise the oblivious email marketing stories on HN, eg the "cold emailing" C level execs whose addresses aren't publicly available. Dirty, dirty, dirty.
(And keep this discussion in mind next time you read a HN story teaching people how cool e-mail marketing is for your startup - what it's cool at is often pissing off your users.)
Also Google took a good approach to that issue - when an e-mail has an unsubscribe link and you try to mark it as spam, you get asked if you'd like to unsubscribe instead. And yes, I sometimes do exactly that.