Capped by items.
For example, you could stack runes, Bloodthirsters, pick someone like Nocturne and Yi with their insane AD steroids, and get close to 800+ AD... And that's the cap.
Tenacity, CDR, and Attack Speed are capped because of how they work - it would be totally possible to get 100% CDR or Tenacity, and if your attack Speed was any higher than 2.5, I'm pretty sure the animations would break.
Of course, MR, movement speed, and armor have softcaps/diminishing returns... So I guess that is close to a cap.