The number one improvement to me is that the game is American subs back in the Pacific theater, just like the original Silent Hunter. No offense to those who prefer the German side of things, but I'm a huge fan of the American sub campaign, so SH4 is a very welcome effort to me.
In regards to the comments about prettier graphics, I say bring it on! Since I did not play either SH2 or SH3, the graphics we've been seeing in recent screenshots and videos, which are lightyears ahead of what SH1 had, are raising my excitment level tremendously. And to confirm this to myself, I just finished playing about an hour of SH1 via DosBox. My goodness, the graphics are bad. Sprites for ships, cartoon clouds and waves, and SVGA resolution (stretched on a widescreen LCD no less). UGH! I cannot wait to see my sub and the enemy ships in 3D; it's going to be fun.
As for gameplay, I can't imagine it being much worse than SH1, and that's still a very fun game, so if it ends up being as good as SH1, I'll be happy. From what I've read, it'll pretty much have all the gameplay features one would need to simulate a WWII sub in the Pacific. I've ready many biographies and books about the American sub force, and have many expectations of what a proper sub sim should be, yet I have no worries that Silent Hunter 4 will miss the boat (Pun intended).
Still, in the end, I'm getting a bit bothered by all this negativity. I don't think it's very proper to damn, or praise, the game before it's released. Let's give the developers the opportunity to succeed or fail on the merits of the final product, not screenshots or videos of pre-release builds.