What investment treaties typically do is offer investors (in either direction) reciprocal guarantees of basic principles such as fair and equitable treatment, protection and security, non-discrimination both generally and by comparison with local investors, and against expropriation without compensation (also guaranteed by the European convention on human rights).
That's from a letter to the editor in the Guardian (the first letter at the link), in response to an op-ed criticizing ISDS in the TTIP. The author of the letter is an investment arbitration lawyer. He concludes the piece by saying:
There are many weighty arguments against the TTIP, as there are in its favour. But they deserve to be debated on their objective merits, not by mythological scaremongering.
That's exactly right, and this what I've been pushing for. In particular, it is the content of these basic principles that needs debating.
I have no problem with non-discrimination. We may quibble about its precise contours, but any reasonable definition works pretty well and is not too controversial. No doubt there are some people who think we should discriminate against foreign investment, but it's a minority, from what I can tell. The majority sees the benefits of foreign investment, and wants to liberalize in this area.
But the other principles are trickier. In particular, fair and equitable treatment remains extremely unclear to me, no matter how much people try to clarify it. I still look at a wide range of domestic law and policy and think, couldn't that be challenged as treatment that is not "fair and equitable"? To take one example: the entire U.S. tax code!
So for me, one of the keys to the debate is whether anyone can offer clear explanations of these basic principles, and whether politicians and voters would be OK with these explanations. That's where the debate should be, in my view, if we are ever going to get this issue resolved. (There are other issues, too, of course, such as: If these are such good principles, why do they only apply to foreign investors and investments?)