It goes without saying that Stephan has produced among the most level-headed and thoughtful posts that there are online. That said, the evidence for his "default stance" here is severely limited and so I'm not that worried.
The only argument he offers in support of his suggestion is:
most of our ancestors have probably
been eating more carb than fat for a
very long time, so my default stance
at this point is that if you're going
to bias your diet toward a
macronutrient, I'd go for carb.
But this doesn't seem terribly convincing. Hunter gatherers have also spent lots of time burning their own body fat and animal protein. That's the metabolic condition that low carb is mimicking (fasting/starvation/winter/calorie restriction). What foods we have eaten is potentially a distraction from what our bodies have actually metabolised. There seem to be a fair few modern studies showing advantages to carbohydrate restriction/ketosis- I think (and have thought for a while, even before this heresy) that Stephan attaches too much weight to what healthy traditions have eaten in recent history. While the high fat proponent has a variety of suggestions to make as to why, at least in certain conditions, restricting carbohydrate and replacing it with fat would be a good idea (in terms of suggested pathways, metabolic effects etc.), I don't know of any comparable reasons to think that biasing your diet towards carbs would be a good idea and fat a bad one. The only suggestions I hear about fat are things about it being highly rewarding (which on its own, it isn't) and containing lots of calories per gram (which if it's important, can be pretty easily rectified).
In any case, Stephan grants that:
Our metabolism is highly attuned to
coordinating the appropriate metabolic
response to differing
carbohydrate-to-fat ratios... cultures have thrived on
practically nothing but carbohydrate
(New Guinea highlanders, etc.) as well
as mostly fat (Inuit, etc).
So I don't think we ought to think that what different cultures have eaten in terms of macronutrient portions (so far as culture-wide generalisations can be made) is likely to be warrant biasing our diet towards one macronutrient or another. I don't think that "The fact that there are so may healthy high-starch cultures, far more than there are high-fat cultures" adds to the "weight of the evidence at all." This may well be more historical accident (for example, more cultures living in warm climates than cold ones in recent times) than a consequence of high carb diets being superior nutritionally (rather than superior for food security, convenience etc.). I also think it's doubtful that there could have been such a preponderance of higher carb eaters over lower carb ones, that we would have since adapted (somewhat losing our flexibility) to be more able to eat high carb diets. The question of adaptation here is also, of course, a loaded one. Maybe we've evolved such that higher carb ratios lead to some advantages (fewer muscle cramps, higher glycogen levels (for running away from predators), easier digestion, higher growth, higher reproduction, higher serotonin levels and so on), whereas long term low carb diets risk the converse disadvantages, but also reduce appetite, reduce weight, reduce cancer, increase longevity. Perhaps we have adapted to have high carb as our historic default- allowing quick recovery, growth, reproduction, shorter life- but still low carb is the diet best suited to our current situation (in the developed world). There might still be reasons to favour a low carb ratio, based on other reasons, however, if it's granted that the humans traditionally eating high carb diets were more active, getting more sun, less stressed, less metabolically damaged, had lower food availability, than we are presently.
The one reason that I do think there is for possibly favouring carbs over fat is that if you eat 85% of your calories from butter or tallow, then you will have basically consumed no micronutrients (apart from lots of vitamin k2). If you eat 85% of your calories from potato, however, you've already met most of your micronutritional needs and gone more than halfway to meeting your protein needs. Of course, it's possible, of course, for low carbists to rectify this situation though, eating low carb vegetables and being selective about what other foods they eat.
Also, although this hasn't been explicitly mentioned, it does seem to be a factor in some people's considerations: I wouldn't attach too much weight to the fact that Stephan remains lean and healthy, despite eating a high carb diet of potato and lentils. Before going paleo, I (aged ~22) lived on mostly wheatgerm and soy milk and I was very lean, very active and so on and lots of my peers are the same, despite getting most of their calories from added sugar. Thus these anecdotal individual cases shouldn't be treated as being particularly indicative.