Everything is already perfectly summarized.
- The [5]% per point is too low value. To have significant damages later on, you would have put everything in retribution and be naked with no other points in your character. Furthermore, the retribution skill is useless for at least the 8 first levels since the enemy damages are low and so is your retribution skill. At those levels, you often do between 1-10 damages with retribution. I really like the previously proposed solutions to add a flat value to the reflect like "Damages=(Retribution rank*5% damages received)+(retribution rank*5)". It would really help during the first levels to notice the impact of retribution in the fight since you would at least reflect 6 damages instead of 1 and the scalling later in the game would not be too huge.
- The last problem is that there is no aggro system for the player to cleverly use. Taunt is blocked by physical armor and has a low radius range. Moreover, the AI will always gank on the less tanky character, avoiding your tank all the time. This issue is way more complexe to fix because it needs some redesign of the AI.