I've said this elsewhere, but 250 characters is roughly 3 fairly simple sentences. Generally speaking, if you can't expand on your experience using a resource to that level, it won't give useful feedback to the author or another user. Sure, there will be exceptions but we're intentionally discouraging reviews that don't include detail. The minimum length may be tweaked over time, but we're still taking about a minimum of 2 tweets.
(For reference, the above paragraph is over 430 characters.)
(For reference, the above paragraph is over 430 characters.)