I think one of the things the pros do that I never really thought of is they get different items depending on whether they are ahead of behind in lane. For instance, Wings (inb4 not a "pro," I know..) got an early advantage as Zed mid so he bought an early brutalizer in order to help him keep his advantage and make it bigger. What are some of the "snowball" items for my type of roles? Here are the main roles I play:
-Tanky support (Taric/Leona) -Poke support (Lulu, Nunu) -Tanky jungler (Amumu, Maokai) -AD jungle (Lee sin) -AP mid
I mean, just common sense as far as your lane goes.
Can you kill someone quick? Buy damage so you can kill them even faster and force them out.
Getting big and having trades? Maybe you actually want to rush a Giant's Belt so you can outsustain anything.
CDR is usually an awesome early game stat. I'd say you're probably NEVER at a disadvantage if you rush something with a little bit of CDR. Brut for offensive AD. Codex for offensive AP. Kindlegem for tankier champs.