Hmm, this is a really great product and I've loved using it, but 2 projects? That's so ridiculously low that 'free forever' seems a bit hollow and vacuous now. It would be better if you just came out and said it. Are you just supposed to go one humungo project now?

I'm certainly not paying a monthly recurring fee for a mock up tool to play with side projects! I could see upgrading it for the collaboration aspect for clients, but for toy and hobby projects you've seemingly just killed the tool.

Calling it 'using in a casual manner' is bad, how is having a project per idea and being limited to 2 ideas 'casual'? A helpful compromise?!?

Personally I think you've focused on completely the wrong thing to go premium for. I'm surprised you didn't go private projects & apps/dropbox integration (standard) > revision control & master pages (pro) > collaboration (ultimate) or something like that. But that's just the programmer in me I guess, we all know that multiple projects won't add to your costs, just massive inconvenience to the free users.

Still, it's your product so do what you want and you're allowed to change your mind, just don't expect a lot of people cheering you on. I'm just personally sad I probably won't use it any more as I'm not going to faff around trying to organise multiple thoughts into one humongous project.

As a reminder of what you originally said when you introduced it to HN:

We'll keep the main functionality free (with some reasonable limitations). We plan on adding some really cool collaboration and annotation/feedback features soon that we'll probably ask a buck or two for.

I just don't see 2 projects as reasonable at all but I always suspected something like this would happen as the tool was just too well done. Note that I would probably buy it for a reasonable fixed price (like $50-70) but obviously the storage, etc. will constantly be a drain on you. It is better than balsamiq, but not better enough to be dipping into my pocket every month for the rest of my life. The problem with saas I guess.

EDIT: I guess this is the problem with an online tool being useful to both individuals and corporations. I can see real value from a recurring fee from dropbox as an individual, but a mock up tool? For what (should) be a desktop app that's gone to a sass model, are individuals really going to pay a recurring fee for this? On the other hand, it's a perfectly reasonable price for businesses, even small and micro ones if you use it regularly. I would definitely sign up if I do more consultancy again. Perhaps I'm cheap as an individual, but to me it's just far too expensive for my personal use case.