1st a round world....
2nd as lukeFF said, Realistic tracking of sinking of historical ships and their sinkings, not just when player sinks them but also appearance dates based one historical data... this could also go for the names of sunk mechants to some extent (generated names only available after they were used, especially important for Liberty ships and others built during the war)
2nd (again) the more historical accuracy the better
3rd (or possibly first) better weather, I didn't feel sh4's weather was an improvement at all
just my 2 cents
