I think the line here is pretty straightforward: the contents of your mind are all yours. Anything beyond that (documents, source code, lists of prospects) is not.

Non-compete clauses will try to limit the usefulness of the "in your mind" knowledge by restricting the domains in which you can work post-departure. It's my understanding that such clauses are generally held to be unenforceable except in an acquisition scenario.

I think the line is drawn at actual stealing. In this case, they're not redesigning the protocol from memory or black box testing. Allegedly they took several specs of the protocol from former employees. Even then, apparently the founders were involved in some of the Patent filings from splunk that they are accused of violating. You cant claim IP for a company in the form of a patent and then turn around and re-implement that IP. You clearly believed it was patent-able since you patented it. There's ways of doing this (clean room dev) if you wanted to do that without infringement. (I do feel a lot of the patent claims in the lawsuit are typical generic weak software patents)

Really egregious is taking the sales data. Business analytics around leads, customer satisfaction, pricing, etc are not the same as retaining general knowledge. If you left and remember the point of contact you had at a customer, that's allowed (barring non-solicitation agreements). If you leave and you take a list of customers, data that the business has generated about them, etc, that was never yours and it's not your knowledge. It's clearly the business's and there's usually dozens of people involved in the creation. That's clearly theft, especially since it was never yours to begin with.

honestly, I've never moved from a role in one company to a role in a new company that directly competes with the role I had in my old company.

While I have jumped to competitors, I moved to roles that weren't in any form competition to my former team/role. That makes it easy, even if I would accidentally take things with me, I wouldn't be tempted to look at it, as there would be no point.

So yes, I take all my growth, knowledge and experience, but nothing that is really unique (say trade secrets) to old company would directly apply to my new role, so there has never been any problem. Once one is willing to jump to a competitor in a manner where you trade secret knowledge would benefit your role directly, one is creating a problem.

To me, unless there is a legal document you signed with your employer, there is no line. Even, IMO, IP is not `property` so that it cannot be used against. But that is another discussion.