The legal risks of a binding contract being formed merely because parties have exploratory discussions are pretty minimal.

To have a binding contract, even a verbal one, you need to have a meeting of the minds on terms of a deal that typically include mutual consideration - if what you are discussing is preliminary and does not include commitments upon which people can reasonably rely, it usually is safe to conclude that there is no basis for a binding contract.

The "false promise" type of situation is a specialized one that normally does not arise in most startup situations. In general, it is a variation of fraud. Fraud generally consists of false representations made to a party upon which that party reasonably relies to his detriment - in other words, getting "tricked," misled, etc. in a way that hurts you. A typical misrepresentation must be one of a fact that is falsely represented by a party (e.g., "my company has achieved x sales and you should invest in it when sales are nowhere near what is represented). The "false promise" variation of fraud is one in which the misrepresentation consists of a false fact but the fact itself is intangible - in this case, a false representation of one's state of mind. Thus, if one makes a promise without any intention of ever performing it, this is in the nature of a false statement of fact by which one can "trick" another person and thus constitutes a basis for fraud (e.g., if you do x, y, and z for my company, I will grant you a 20% interest in my company where the person making the promise never had any intention of performing on that promise). This "false promise" type of fraud is not the type of promise upon which a contract is formed but rather one upon which an action for fraud is predicated. In the contract context, it is most often used to try to get out of a contract ("I know my contract says I must do x, but the only reason I entered into it was in reliance on a promise made by the other party that he never intended to perform - because of the promissory fraud, I can rescind my contract").

With respect to promises or exchanges upon which parties try to claim that a partnership has resulted, you need the essential element that any such venture has to be based on the idea of splitting or sharing profits arising from your joint activity in some fashion (almost always in some percentage fashion). Thus, this represents only a narrow category of situations where one might inadvertently find himself faced with a claim by another party that negotiations they engaged in allegedly resulted in a legal partnership between them.

As appears in the TC case, the "false promise" and "binding partnership" type of claims tend to arise when parties have had complex dealings between them, allowing one or more of the parties to assert colorable claims of having been misled to one's detriment or of claiming that the parties agreed to undertake a venture by which they would split profits.

In 99.99% of cases, you can safely undertake typical negotiations with another party without fear that you will inadvertently enter into a contract. This is one aspect of law in which, surprisingly, common sense is actually your best guide. Contracts ultimately tend to turn on reasonable expectations of the parties and, if what you are doing does not feel like a binding commitment, it usually is not.

I don't think you'd have grounds for suit for just exploring an idea - but TC and Fusion Garage did a whole lot more than just exploring.

There's no obvious line to draw, but certainly there's a real difference between talk and a whole lot of work, and outright fraudulent behavior, which (from TC's account) is what happened in this case.