You had problems with sieges? Maybe it was because I made the mistake of only playing on normal, but thus far the enemy has been completely unable to put forth any defense against my flying airships of doom that do ~50 damage a turn to cities. (You need a population 3 undead city to make them if you do the buildings right.) Also, my ancient liches or whatever it was I got from the temple I built in my third town (for a nice 700 gold cost...) are doing a number as well, with their 3-range attacks doing something around 80 damage now.
One of the main problems I've run into is that there doesn't seem to be a 'disband' option for units.... and when some of the super-powerful units cost 15 or so resources a turn to maintain (Not counting the extra 8 or so mana for the buffs I have on them), I run into issues. A bug I found in that region is that if you put them on a transport ship they stop using their own upkeep cost. Also, in the resource section, the penalty for insufficient food production is lower pop gain in your food-eating cities. Not negative gain though, even at the -20 food/turn I've been maintaining recently. (darn blade-masters eating 9 food a turn)
As for 'cool new spells', I've not researched many, but I've found more (In ruins and the like, or from doing god-quests). Water walking was pretty much necessary for crossing the kracken-infested oceans. An powerful elemental resistance buff god-spell (something like +60) made me regret doing another gods mission and losing the favor required to cast it. A few summons I've found are neat enough to use (something comparable to flying imps. Teleport sounds like the most useful spell in the game, but it doesn't appear to actually work. And finally I have a 2-spell kill combo... some 15 elemental damage + frozen debuff spell, and a god-spell that does some 18 or so physical but also shatters frozen units.
My biggest complaint about the game as is is the lack of casting-power progression. In MOM you could spend your incoming power on your casting ability, which allowed you to cast more MP worth of spells a turn. I've thus-far not seen any progression at all to my casting speed, and I'm now bringing in >100 mana a turn. It always becomes a struggle to decide if I should spend the turn buffing this super-unit of mine or that, or if I need it for combat against something big, or to heal one of my super-units, or to start the 6 turn process of creating a new super-unit. (Since it takes around 6 turns and 800 gold to get all the useful available buffs on a unit now. )