You used to be able to see the number of unique traders a question had next to the liquidity that a question has. (It's still there when you click on the question).

I think this is very useful information because it tells you how seriously you should take the prediction of a market, whether you just want to know if it's a reliable prediction or to give you second thought before you bet against the consensus.

The liquidity doesn't tell you this, especially now that it doesn't go up automatically as new traders trade on a market.