Everyone has a different criteria while rating things. Their own one. It should be better to have a common ground to vote.
What does it mean when someone new to WP rate an idea with 5?
How that compares with an advance designer/developer making a 5 star?
The value of the 5 stars is the same for both?
How many 1 star are needed for the idea to be consider for implementation? How many 5 stars?
Now, rating an idea is like a vote for it. It should be better to have 50% system, I vote or I don't.
Or to still have a 5 stars system I propose something like this:
1 star : I like it, but I can live without it
2 stars: Good idea for a plugin
3 stars: It will be very helpful
4 stars: This must be a core feature
5 stars: The whole community benefits from this