What did MongoDB not being ACID compliant before v4 really mean?

Question

I am not a database expert and have no formal computer science background, so bear with me. I want to know the kinds of real world negative things that can happen

Answer 1

I think other people gave good answers already. However i would like to add that there are ACID NOSQL DBs (like http://ravendb.net/ ). So it is not only decision NOSQL - no ACID vs Relational with ACID....