Their key component was in their "Command and Control / Fog of War" mechanisms. All Generals and Brigadiers had "personalities' (Careful, Steady, Aggressive and Political) . . . and how their personalities interacted (along with a dice roll) affected how they interpreted their orders once they made contact with the enemy.
They might follow their orders as intended; or interpret them more timidly or more rashly than intended; or they might just halt and wait for new orders (particularly with a "political" C-in-C).
Also, whenever their orders are changed, they had an "initiative" roll they had to make before they could interpret and act upon their new orders.
Together these created a lot of interesting situations on the tabletop. Also it forced the C-in-C to really consider which Brigadier he assigned to which task and what orders he chose to give them . . . in other words, the sort of command decisions that real generals had to make.
I would like to carry these aspects over to my ECW rules. But my opponent has been traveling for the past month and I've not had a chance to discuss it with him yet . . . still I do not foresee any difficulties.
My shooting and melee mechanisms were not particularly unusual and seemed to work well (and I think that most rules over-rate artillery's effectiveness in these earlier periods . . . so I've made them moderately ineffective).
One thing that I do plan on changing from my "Tricorne Wars" is movement (instead of IGO/UGO). I recently (during an extended illness) played some solo "Victory Without Quarter" rules. I particularly enjoyed the card-motivated actions . . . but more for brigades than for individual units.
I plan to use a card-activated system. However it will not have individual units, but only "commands" (i.e., Generals and Brigadiers). The number of cards they have in the Action Deck will depend upon their individual "Initiative" ratings . . . and, of course, even so the "End of Turn" card can turn up any time.
Also I'm looking at concentrating on the early part of the ECW (say 1642-1645) during which a high percentage of units were rated "poor" and very few could be called "superior" . . . which once again will require C-in-Cs to think about what units to assign to which Brigades.
Anyway, those are a few of the things I'm tossing around.