Fun is
--cool variety in factions. Everyone keeps saying it but nobody at SD seems to be listening. Maybe a better backstory? Not. More lore? Not. Different kind of armor? Not. Simple answer: Different units per faction. Fantastic units per faction.
--when you take over another city you get to make their units. This is fun because there is a REAL reason to take a city--you get their cool units, not their gildar (dull) and metal (duller) and same old units (see above).
--Magic where you get to choose some of your spells at the start (beta-fail), have a systematic improvement path so you can get the spells you want (beta-fail), and where after 300 turns you're using different spells (beta-somewhat fail)! Kudos for slightly improved variety, but many times after 300 turns the Flame bolt is still one of the few useful spells cause they can't resist your 'slow spell'.
--fantastic units (summoning, building, finding, don't care). Yes, build men/trogs/whatever into units. Their advantage is they have metal weapons and armor and are plentiful and work in units. Fantastic units turn the game from armies with magic and whacking to a magical world with whacking.
--tactical battles with choices that actually mean something. Whoever decided the tacticals need to last less than 3 minutes is ignoring a fun part of the game. IF they were really fun I'd agree, but without the tunneling/flying/etc., it's just not fun. Add something guys, they're ug.
Adding fun to the game is the problem, nor more logic or details or armor or metal. As it is, it's not as fun as AOWSM, which was still a far cry from MoM.
p.s. It's a lot better than Elemental, but compared to other available games vying for our entertainment time, this still isn't sustainably fun.
