Why none of the soldiers are caster? the battle can be more epic if there are involvement of spell casting.

image this, warrior with melee in frontline, archery in mid long range and have mix with few mage as support. now you can see lighting, fireball, ice spike flying around the whole battle field. the whole battle can be more epic.

Another thing is i wish there are more large scale fight between two side. like both side have a big troops stand in open field and both charge into each other. because the whole quest line is like "ambush this, now the fort is your", "ambush that, now the fort is your". i am more hoping the enemy side will launch counter attack to try take over certain area and you do the defending

Some of the officers DO cast spells, but it seems they only cast them at dragons. As to your second point, there is one.

you mean whiterun? this part is a bit dull as it is more like barricade rush

The problem with magic would be all the friendly fire. Especially once you get past the apprentice spells and into adept, expert and master spells.

Could you imagine a half dozen NPCs firing exploding fireballs everywhere? ha ha that would be hysterical. Everything would be dead in seconds.

na, not having half dozen of mage. it is more like 1 or 2 mage mix with few dozen of soliders (remember most nord don't like being mage, but still there mage exists in skyrim)

Well, lag increases with an increase in the number of actors the game has to keep track of. While I would love an epic battle (a la Hollywood), it would probably be stop-motion action for my bitty computer. Whiterun dropped my FPS down to single digits for much of the battle and I didn't know what the Oblivion was going on.

Plus most medieval battles where quite small anyway