There are a lot of tooltips in league that are missing key information.
Rumble's Harpoon doesn't tell you its actual CD (only the time between harpoons, not the time to stock another one), Kalista's Rend doesn't tell you that multiple spears do more damage, Ahri's Q doesn't tell you the haste amount or duration, Alistar's tooltips don't mention that his combo even exists... there are more but you get the point.
Going past the numerous individual failings, there are several systematic problems.
* You can't see how your abilities upgrade unless you have an unspent skill point.
* Every spell with a static CD fails to mention that it has a static CD.
* Many "out of combat" effects fail to mention how long you have to be out of combat.
* AA-modifiers fail to specify when they work on towers and when they don't.
* Ranged spells fail to list whether or not they count as projectiles for windwall/unbreakable, which leads to clarity issues at times.
A player who is new to a champion should be able to read their abilities and understand what they're going to do, but as of now they have to go on the wiki.
A first time Rumble might think "Oh shit E has a 0.5 second CD! I'll take it at level 1 and all-in!". The new player experience is bad enough without the game hiding information from them.
As a side note: It would be nice if we could read the tooltips of other champion's spells. New players could understand what their opponent's champ does without tabbing into the wiki mid game.