That's what makes the game fun for those who can think... cuz the first time I had thornmail used on me... well yeah I ended up shredded just like that, and was pissed cuz I had 3 bloodthirsters doing 75% healing, so it should of more then countered the thornmail but didn't... but then the second time, I ditched one of my PD's, grabbed megaton cloak instead, switch out infinity edge to madreds ... and oh dear thornmail is healing me! (2xbloodthirsters charged fully lol) ...
How long have you been playing?
It seems like an awful lot of information to process and know about to make the right decision. Conceivably folks will have good knowledge of their champs spells.
When it comes to which gear fits which playstyle most people already fail or barely manage to read up on some guides. Then you start to talk about either adjusting your playstyle based on what champs you are facing or counter their play. This requires knowledge of what those champs do and the capacity to instantly decide what to do about it yourself.
Past that you are looking at the gear the hostiles have and make decisions to adjust your own gear accordingly. That's a level of complexity that requires fairly intimate knowledge of pretty much all gear there is and the ability to make those decisions in short amounts of time.
While this in general sounds like something that could be fun I can't help but wonder whether the time required to acquire all that knowledge is outside of "casual gaming".