Subscriptions are a funny thing. But first, I'm an AdBlock user, and I recommend it for my clients (
http://www.robsheldon.com/adblockplus).
If Reddit immediately offered a subscription service and changed nothing else about their site ... I probably wouldn't subscribe at any price. Reddit is a bit of an occasional guilty pleasure for me, and if it went away, I would write that off as one less distraction. I'm aware that Redditors on the whole have accomplished some pretty great things in their own right, so something like that ought to exist, but I just couldn't justify the cost to me.
BUT! If Reddit required a paid subscription in order to post a comment or story, I would actually be more motivated to spend money on that. It would dramatically change the nature of the site, into one that I think would be more attractive -- to me.
That's just me though, and I'm not a very good example of any kind of business model. ;-) (Nor do I think Reddit actually should do this -- it would be a bad business move.)