For old CMBO veterans, CMBN will seem familiar. The CMx2 engine adds some substantial changes, but many things haven't changed at all. Here is a listing of some of the things that did change.
CMBN has 1:1 modeling of everything, particularly infantry units. This makes infantry units seem much more realistic, as compared to CMBO infantry which moved as a token.
CMBN has no automatic map generator. Quickbattles still exist but must use a pre-generated map. The nice force-picker for quickbattles found in CMBO is gone, replaced by a force picker that happily generates a force of all tanks if you allow it armor.
Woods don't provide substantial cover. (In fact, there are no "woods" as such in CMBN; there are only trees, and two base-level terrain types that offer concealment.) A unit can hide in (or behind) trees, and by breaking line of sight be safe. But then it cannot fire either, because it can't see. Or a unit can be where it can see, but then it gets little cover from the trees.
Most common buildings provide little cover. Also, buildings have their doors simulated, so you cannot always safely enter from the nearest side. Sometimes your pixeltruppen have to run out in the street to get to the door, in awful exposure.
CMBN has a "realtime" mode, which is just what it says. Both players have to do everything in real time. There is no playback of turns because there are no turns. You can give orders at any time.
There is no wego TCP mode. You can play the AI either wego or real-time. You can play a person wego via email (or hotseat), or you can play a person in real-time mode via TCP. But unlike CMBO, you cannot play someone else wego via TCP. There is a workaround program called H2H Helper that makes email similar to TCP.
The ambush command is, sadly, gone.
Light mortars no longer suck. To the contrary: they are the most powerful anti-infantry weapons on the battlefield, when fired direct. Also, they are quite mobile, even the 81mm.
Unless fired at a TRP or pre-planned, artillery now requires observation of spotting rounds. Thus it is very difficult to use the old CMBO tactic of keyhole sighted artillery.