Call to Action: Coming from other Engines

We currently have a Coming from other Engines document that is targeted at people new to Core but familiar with other game creators or engines, offering an overview over the common things differences.

We are now posting this Call to Action to collect feedback on it, so please let us know what your most important "gotcha"'s were when you moved from a different tool to Core.

What was the most important difference? What was really similar?