Simply put, the lack of itemization options is a problem across the entire spectrum of champion types.
Are you an APC? Build the following:

There are a few exceptions to this, and it mostly has to do with what champion you're playing (Fizz would want a Lich Bane for example), but that's largely the APC build right now. (not necessarily in the order listed)
ADC? Build this:
[
or
] [
or
]

Again, some champions (like Ezreal or Corki) would use an alternate route involving
, but that's Champion based, not situational.
Playing a Tank? This is where it actually gets semi interesting... here we go:
Against all AD:
(if they're auto dependent like Master Yi, Vayne etc)
(If you can make use of the passive and it doesn't throw you over the CDR cap), and you can pretty much finish out with a
for the giggles (don't forget your boots too).
Against AD lane opponent with a mix of damage on the enemy team?
(to deal with lane opponent, it's cheapish, and gives you that extra little burn while you're tanking stuff), [
or
]
You pretty much pick and choose situation as to what defense you need once you get out of laning phase, but the options are relatively limited.
You probably get the idea by now... there aren't MANY build options for most of the classes in League. You will even notice this while watching LCS (or LCK, LPL, LMS etc) if a pro player builds something that isn't in the "usual list" the casters will almost always say either "I don't understand why they're building that, this would be better" or, "Well that's interesting, we'll have to watch how that item works out for him this game."
Just my opinion, but I feel there are a lot of gaps in the item-lines that should be addressed.