1) The idea seems to have traction. You've figured out how to scale the back end. You've got all the pieces in place to grow this into a business, if that's what you want.

I'm not certain what sort of business model you've been thinking about for this venture or if you were just putting it together to play with it. If you want to make money, start charging for it. Test the waters by asking what your existing customers would pay for something like this. (Consider telling them the best responses will get free lifetime accounts or something similar so you minimize the "I'll give a low price so I don't get charged too much down the road" thinking.)

The price doesn't have to be right, but pay attention to what your customers do as you raise and lower to find a sweet spot. Give your existing customer a sizable "thank you" gesture when you announce pricing. See how people continue to use your service and how they continue to convert. Figure out which demographics each individual falls into and try to understand how to tap the rest of the market that they represent.

Once you have a good idea on what your target market is, search engine marketing and a solid social-sharing campaign might be a good tactic. Since Dropbox customers are already familiar with the "give this to friends, get free stuff" model, this sort of sharing campaign may convert better than others who just copying because of its popularity. Try to connect your sharing campaign into Dropbox's sharing model/message somehow for some emotional influence with customers who already love Dropbox. Think "For each of your friends you brought to Dropbox, we'll match the love! For those who start using SaveMyInbox, we'll double it!"

I have some other ideas, but this is the general direction I'd move in if you were trying to monetize it. If you're not interested in running this as a business, then flip it and move on. This is assuming you've met all the goals you were trying to meet with this project. Drive it as long its satisfying (and fun).

2) I don't necessarily consider myself the best software engineer either, however the demand will require B and C-listers, too. So if you want to get better at development, it never hurts to keep up with it. It'll be easier now to get a job as a sub-par programmer and get better from the experience than it will be once the market shifts the demand away. Just understand that by taking this route, if you don't improve to the levels that your employer expects, you might quickly find yourself out of a job. Further, you're only 24. Granted, we're seeing rockstar developers at the age of 18, but that's not necessarily par for the course. I'm pushing 30 and I'm definitely not cut out to be an awesome programmer EVER (because the younger crowd has more time available to accelerate past my current speed of improvement). So I make my strengths up in my diversity of skills, my ability to work with and for people better than the young cowboy programmer, and to be better at recognizing opportunity to employ my assortment of skills and improve myself while remaining in the workforce.

I don't mean to side-step your question, but I hope my thoughts will help you think differently about your situation. It's not just about having to adapt yourself to future conditions, it's also about taking advantages of where you are now (physically and spatially) going forward. Regardless of what field you prefer to work in (you're working in your preferred field, right?), you will be a commodity as long as your work makes people want to collaborate with you. Bottom line. :)