I've told lots of people their startup ideas sucks. For a time, lots of people asked me for my opinions and advice on their startup ideas, from a context of "I want to raise seed funding, how do I do it".

Here's a few phrases that let people know their startup idea sucks without being judgey on them:

- How did you validate that there was a market for this? - How many businesses have committed to pay for this? - This sounds like a 2-sided marketplace, how are you going to solve the chicken-and-egg problem? - Sounds cool, but consumer is really hard. Are you sure you wouldn't prefer to build a nice B2B startup instead?

The last one is interesting. Lots of people have ideas that are "like facebook but different". Many companies like that have succeeded (instragram, whatsapp, twitter, secret). But those companies are very very hard to build, and thousands of startups like that fail. So its not so much that their idea sucks - as it might be really cool - just more that they might prefer to build a different startup in a different space, especially if its their first startup. Nothing says early validation like 50 people who give you their credit cards - not easy in B2C.