1 - Never change the rules at the middle of a "game".
2 - How can you have someone hashing at your pool, with a probability of having to hash for more than 5000 hours (7 months) before getting ANY reward, only to have the chance of winning a extra Bitcoin (less than 73€)?
That said, I liked your pool before the DDOS attack, I still like it (till now I hashed 228547 shares for you) but at the present hashing rate, I would be better solo hashing, as the reward would be to get the whole 25 Bitcoins with some (low) probability of getting those in less that 7 month).
It's not with this type of attitudes, like forcing the miners to hash exclusively for you ("you must have been actively mining at the site when the first block is discovered"), that you'll get back the "big guys" that were hashing for your pool before the DDOS attack.
Good luck to your pool and best regards.
Rui
To address your first concern; the rule change was merely to protect miners who were honestly contributing to the site from those who were just trying to take advantage of the contest.
1 - Never change the rules at the middle of a "game".
I think this falls under the term "Growing Pains". There will always be unintended consequences. People can't think of every possibility. It would probably be best to declare the site in Beta Testing and scale back rewards until it is seen how it performs in the real world.
ct1aic nailed it on the head, when ever you start a project, there will always be unforeseen obstacles, problems, and set backs that you for some reason or another over looked or thought of.
As for your second concern, I understand 1 Bitcoin is not much, but keep in mind I have limited resources, and as it stands, I myself have ever owned at most only 2 Bitcoins, and I am offering half of that away. I know its not much, but its an incentive to hopefully get some people to join the pool.