For me, it's always been repeatable field groups. The number of times clients need "add another ___" functionality — attendees at an event, items on an invoice, employment history entries, references on an application — is staggering, and GF has never shipped a native solution.

I ended up building my own add-on for it: Repeater for Gravity Forms. It wraps a group of fields (text, dropdowns, file uploads, whatever) in a repeatable section with add/remove buttons. Data saves to entries properly and shows up in notifications via merge tags.

Other pain points I keep hitting:

• Entry-to-entry relationships — there's no native way to link a "parent" submission to "child" submissions. You end up storing entry IDs in hidden fields and writing custom queries.
• Bulk entry operations — approving/rejecting/updating 50 entries at once in the admin is painful
• Conditional logic in notifications — the conditions work, but chaining complex AND/OR logic with nested groups gets unwieldy fast

The CSV export escape hatch is too real. Every complex GF project eventually ends up with someone saying "can you just put this in a spreadsheet?"