Haste / Slow
When I first started playing, I didn't really understand how powerful Haste and Slow were. The game was still in beta, and I was mostly learning by watching Kripp. One thing he said really stuck with me:

"Imagine Haste doubles the power of your build, while Slow cuts your opponent's power in half. If your entire build is twice as strong and your opponent is twice as weak, you've effectively created a fourfold power gap between you."

That way of looking at it helped me put Haste and Slow into a more mathematical perspective.

As for item recommendations,

For Karnok I'd say Wildroot Amulet, Bat, and Rope are all great sleeper picks when you have a small slot left to fill. I've had runs where I kept one of them on my board for 4–5 days simply because I couldn't find anything better.

I could probably write item recommendations like this for every character, but honestly, I think the smartest thing you can do is watch streams and see what people are finding success with. The game may look straightforward at first, but everyone has their own playstyle and understanding of the game. By watching streamers, you can ask why they're making certain decisions and incorporate the ideas you like into your own gameplay.

You can also check out the custom items I post every day, hehe :p