Understanding that there are Reverse Dictionaries, they aren't very good. The proposed sight would be ideally for people to get input for a/some concrete word(s) that could define the idea that they have searched for.
e.g. Question: "Words that describe how long someone is sick before they are show symptoms" Answer: incubation
Obviously these answers are subjective, but considering people are better at understanding/translating "ideas" than computer's it'd be better than a reverse dictionary.