You'll get a new historical environment, with very detailed OOBs and campaigns. For many this is sufficient, but this is not all.
You get also some new mechanisms & features. Right, you will perhaps not feel the same quantum leap between AACW and NCP compared to what happened between BOA and AACW, but still, take a mn to read the novelties below (in no particular order):
I - Rules of Engagments. ROE are 4 new buttons under the postures of your stack. You can now specify if you want an all-out attack with your stack (no retreat checks for several rounds, losses increased to both side, etc.), or a probe attack, or in defence if you want to fight to the death , retreat if threathened, retreat as soon a shot is fired, etc.
II - New "Napoleonics" themed concepts: Cavalry can now charge in battle (involving a morale check), if it does, will try to cut down weakened or disrupted elements. Opposing Infantries can try to form in square. The Guards units can now commit themselves, in masse, upon the order of some special leaders (Napoleon, Bessieres, Constantin, etc.) and if they do, all your side get a bonus and can litteraly rout the enemy in no time. Except that if they don't, beware the counter-effect you take.
III - 25 new abilities, like Boarding (Nelson), Genius (Wellington and Napoleon), this allow them to 'handle' 10 to 15 abilities, contrary to a max of 4 in AACW. Offensive or Defensive terrain specialist (alter how much troop you can deploy during battle) and more.
IV - Purely cosmetic, but cool anyway: An alternate display, called '3D sprites'. You must have seen then in the screenshots.
V - New special terrains: Mountain Crest and passes, Alpine, major road (functions directionnally, a bit like the railway in ACW, ie you have to enter and follow them from one side of a region to another side). With associated new abilities (mountaineers for example, or even Colonial Leader).
VI - New concept of auto-Garrisoning. Now when you take an important location in NCP, depending of your replacement level, a small garrison will automatically appears, sparing you the burden of splitting one of your brigade for this purpose (less hassle too!). Garrisons eat on your replacements as you can guess, but the rule is optional, so you decide! Also they are themed to the region you have to garrison. Liberate Poland and you will get Polish garrisons eg.
VI - New concept of hardened attrition. Some people want to suffers, we heard them
This rule, which is also optional, allow you to 'enjoy' a more realistic attrition rule, where even non moving units can lose soldiers. Also with such a rule, you will need to withdraw troops to depots to get replacements. That will be hard for the French when deep in Mother Russia
I advise to use this rule in PBEM by the way.
VII - Historically themed options. A bit like the various options of AACW, but much more powerful and versatile. You can 'buy' replacements or even units, order new supply to be sent from home, postpone the end of the scenario even, get a bonus in forced march in a given area, augment your chance to have your leaders activated, etc. They vary between scenarios for a good part of them, and yes both side get feedbacks about the use of them. These options are in additions to custom made events for each scenario as you can guess.
VIII - TCP/IP utility, for whose who don't want to play by PBEM.
IX - Scorched Earth rule. Active in Russia and to a lower level in Spain. This is a rule benefiting a faction, against all enemy of her. The rule can triggers several things, like some supply burned when you take a region, or the structure destroyed (depot burned, village reduced in level). There is also a chance that the region become pillaged. It has a synergistic effect with the Hardened attrition rule, as it acts also as a magnifier to the losses from attrition.
X - New naval order to intercept adjacent fleet, for a better naval game. Improved Naval AI too.
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apy 
