Possibly "bad ideas are cheap". A lot of people got tired of "ideas guys" who never build anything themselves. The best case of a raw idea is an uncut diamond, it will inevitably require work.

The other half of the one-liner is "Ideas are cheap. Execution is everything."

- E.g. an idea is "Everybody should have cheap housing! Or an even better idea, everybody should have free housing."

- Ok, how exactly in concrete terms do we actually do that? (The very expensive Execution of the idea.)

- "Uh, well, I leave that as an exercise to the reader."

Ideas are easy and execution is hard. That's what jaded people mean when they say "ideas are a dime a dozen".