Dunno if I agree with the complexity of the game. I think it's a piece of cake compared to a lot of wargames out there. But perhaps I've played it too much to look at it from a beginners point of view.
It is heavily supply focused, a great deal of the game involves managing and tracking supply, which is physically moved around the map. This is much less effort than you might think.
It is relatively straight forward compared to tactical games, but it is still heavy. The biggest hurdle to learning OCS is overcoming the sheer mind boggling depth that comes from a large scale and three dimensional supply system.
It provided almost unparalleled historic feel and very deep game play. The sides feel very different to play, but this is accomplished very subtlety without the heavy hand of game mechanics being apparent (largely through force organization).
Baltic gap more specifically is a heavily Asymmetric game, the Germans lack supplies and men, but the fighting takes place in heavy terrain and they have some excellent units. Despite the overwhelming superiority in numbers the Russian, there is opportunity for german counter attack and the result is a tense game, where both sides have to watch their moves.
Baltic gap features several very innovative mechanics in addition to OCS's wonderful arsenal of mechanics. In particular there is a catch up mechanism and a cool VP timetable adding yet another layer of decision making.
When I release my podcast hopefully in a few weeks about OCS and baltic gap I'll give more, but I wanted to fill in a few details.