"Lost" bitcoins would just cause all of the other bitcoins to effectively increase in value. It's really only a problem for the person losing the bitcoins and a (small) positive for the rest of the bitcoin community.

It's conceivable that most lost coins will only be lost until the keys can be cracked. Bitcoin keys are extraordinarily difficult to crack right now, but at some distant point in the future, it's conceivable they could be cracked.

IIRC, there are some bitcoins that have been irretrievably lost because it was sent to an invalid key. It's a very small number however.