we just know about ONE straight limitation so far, and this is the players sub.
and there are also some other, like limited interaction (none, i guess) with other boats, limited interaction with bdu and a limited, player affected campaign, which ends in 1943. BUT these limitations are not stated as unchangeable so far!!! so, lets hope for a good base of modability and we will have the best sim we ever wanted.
if not, its going to be like a hamburger. nice to eat, but fast eaten up and you´re getting hungry soon after eaten it.
we will see.