What your really describing, although a great feature is a massive undertaking.
You don't see too many mods that do it because of the sheer complexity involved.
Every post now has to keep track of every user who liked and disliked.
The phpbb_posts tabe would need reworked.
All code in the board that uses this table needs reworked.
All templates that use it and all files that use it.
phpbb_users might need some work.
I cannot describe how many php and template files this board uses to run each day.
A note about those mods - even with the auto installer, mods rarely ever install without hundreds of errors. Especially on our system, as we are deeply customized. The last one I intalled is still broken -.- and largely irreversable
What were honestly hoping for, is for PHPBB to include this feature in a future build. That would be the meal ticket.