Lost Bitcoins is a good thing actually. They make your unlost bitcoins more valuable. (As long as the lost ones aren't yours of course)
I don't think so: nobody knows anybody has lost his coins, there's no way to know it, apart the case I exposed in the previous post, or by admission of the user himself, which can be fraudulent.
AT THE MOMENT the value of your coins is UNAFFECTED by lost coins.
In the future, when we'll hit the wall, THEN this may affect it, as less and less coins will be in circulation.
This is it. You don't know if the coins are truly lost as they are sitting in an address to which someone may or may not have access. Down the road in many years it may lead to more valuable remaining coins.