At what point do you feel it's ok to pay for a subscription for a website? What sorts of things must the site offer you upon subscription that you'd be willing to pay for it?

This is very general, not specificacally a product or anything. I've seen things like, "no ads if you subscribe!". I mean, is that enough to get people to pay for a monthly subscription? What other sort of general ideas are there about paid subscriptions on websites?

Personnally, I've only ever paid for one site in my life... and that was learnvisualstudio.net (for all those videos, such a great deal). But even then, I paid for maybe 3 months and then stopped my sub.

Are there any websites that delve into the customer'd mindset and their reaction to paid subscriptions for websites? I'm sure others have had to of been concerned about this before...

Thanks in advance!