I would say significant stock is at least an amount that doesn't make someone think twice about the package. If the founders are holding 80%, then everyone else is pretty much an employee not an owner.
There's nothing wrong with being an employee. There's nothing wrong with working below market rate in exchange for some equity. Just be clear that it's a job and the only way you'll achieve market rate compensation or above is with a significant exit, limited dilution, and good-will toward you on behalf of the major shareholders.
Completely understand that, but my question was about significant from an employee perspective as I mentioned in the OP.
Is the value of 2% of a startup meaningful compensation? Probably not. See my valuation elsewhere in this discussion.
It's not the $100,000 in salary over three years you didn't get in exchange for taking equity after a deal which made the founders rich if not wealthy. Significant equity aligns your outcomes and interests and gives you a seat at the table when liquidity events are going down.
Significance is more of a sliding ratio. 0.5% is significant if the founders each have 6%. 10% is significant if they have about 25%. There should be only one qualitative step between stakes, and that step should reflect the size of the company.
A final tell on significance is if the ownership structure is relevant to the discussion. Significant equity alters it in a...wait for it...significant way. It signifies that the offer is about more than a hiring decision. If the next programmer passing the interview process would get the same offer next week then the equity isn't significant. Gates and Allen wanted Ballmer, nobody else was going to get 6% with an option to 8 because there was nobody else.
Good luck.