A few quick thoughts after my first ECW Parliament campaign:
Both sides will find that their main force is locked into protecting or threatening a capitol. Oxford and London are close enough that it is almost impossible to shift forces elsewhere without opening your heart to a game losing thrust. Because of this, your main army squats on its heels while tiny, regional armies maneuver with great impact in the North, West and South.
Across the entire map, starting loyalty is so low, that cities must be garrisoned if one is to receive any benefit from them, ie money and VP. Just using 1 small unit in each town containing a requisition office will swallow up a large percentage of your mobile force.
With historical attrition, winter and lack of supply are real killers; if you want to have a field army come spring, cease movement and go into winter quarters in an area that produces adequate supplies.
ECW provides a large number of leaders with solid skills. After a year or two, there are enough excess officers to provide leadership for the garrisons sprinkled around the map.
Be prepared for, from the start, for a massive cash shortage, Each turn, even in the best of times when you control the entire map, there is not enough money generated to purchase more than 2 to 3 replacements chits, or to buy 1 to 2, new, small units.
edit: upgraded to 1.03; Both maintenance and units now cost much less. This is a real game changer and will allow the creation of regional armies larger then a few small units.