Lee Sin's ultimate needs to have its initial 0.25 second stun changed to a root
Lee Sin's ultimate has a small 0.25 second stun effect that occurs before he actually kicks you. This adds a lot of invisible power to the ability because it means your enemy can't use self-shields or other similar self-preservation effects during that wind-up.
The original intent of the mini-stun was to prevent enemies from repositioning with Flash or other abilities before Lee Sin kicks his target. However, because Riot made it a stun, it has the aforementioned side effect of disallowing self-preservation effects from being used. Thus, Lee Sin ends up getting far more kills with his ultimate than he should because his enemy cannot use something to block or mitigate the damage. Granted, the very brief period before Lee Sin kicks you isn't much time to use a defensive ability, but for people who are trying to be reactionary, being unable to block/mitigate is extremely frustrating.
To that end, I would like to recommend that Lee Sin's ultimate have its initial stun changed into a root effect. This would prevent movement abilities like
, but would not prevent things like
. This preserves the original intent of not letting an enemy reposition before the kick, but would allow you to potentially save yourself from the burst damage. Given Lee Sin's already strong mechanics, I doubt this would affect his winrate all that much (and if it did, he could be given compensation buffs).