One thing you could do which should be easy is send chaff. Randomly send out connection requests to some of search engines from your server even though a user is not requesting the data. It makes tying back connections to the users more difficult because you dont know which request is real and which is fake.
SSL would eventually be important because it would protect against man-in-the-middle attacks. Someone could hijack connections to your server claiming to be you and then get all of the requests. Users could potentially be putting in very sensitive information so this could be a big deal. There will also be protection from someone sniffing inbound requests that come into your server as the channel is encrypted.
I understand the expenses thing, so I wouldnt worry too much about that. I'd prefer your service be free and not use SSL than to charge for usage. Although I wouldnt mind some ads, you could monetize a bit on that if you wanted..
The idea of a chaff is interesting and It wouldn't be too difficult to implement. Thanks for the idea.
Also, another feature I was thinking of adding was an ssl option so you could securely access the site. However, as I don't make any money from the site, it becomes more difficult to justify additional expenses.