The complexity that you speak of only existed while standing in front of the spellmaking altar. The actual combat you explain is still equip awesome spell for this situation and left click. You will still be able to do all of those things, but now it will take a few spells. Now you will be able to hit them with ice first (to slow them down), prepare a fire rune on the ground, hit them with a fire weakness spell, have them walk onto the rune, and watch them burn. To me, that is a lot more fun than equipping one spell that lowers their speed, makes them weak to fire, and burns them.
Mmmm... see.... this is the "button-mashing" part of it I mentioned (and granted, that's probably not the most accurate phrase to communicate it, and I apologize to those who were misled by my poorly-chosen words).
I've never been a big fan of fighting games, for instance, because I just have a hard time remembering the button combinations and repeating them correctly in the heat of the moment. I really don't want to see that sort of mechanism as a part of spellcasting in Skyrim.
To use your example - this is how it's likely that it will play out for me -
Hit them with the ice first (to slow them down)
Intend to prepare a fire rune on the ground, and accidentally push the wrong key, and instead equip a healing spell.
Panic as the opponent moves closer and the slowing effect of the ice spell is already wearing off.
Jump to the fire weakness spell and get it off just in the nick of time.
Realize that I skipped right over creating the rune on the ground, so now the slowing effect is gone and the fire weakness is running, but there's no fire damage to take advantage of it.
Die.
For myself, I'd much rather prepare a spell that does all of those things (more or less), aim, click once, then just watch it unfold on its own.
In part, it goes back to the way I look at RPGs - I'm not in the game. I'm the guy who's pushing keys to move around the person who's in the game. The things that person does depend on his/her skills - not mine. If s/he's a tremendously powerful mage, that's reflected by the way that s/he handles magic in the game - not by my facility with pressing keys in the correct order at the correct times.
And in part, honestly, it's just that I sort of svck at the pressing keys in the correct order at the correct time part of it. In an RPG though, in my opinion, that shouldn't matter. That's part of the point, at least to me.