Bill gates is impressive. It's a hard subject to stay objective with. The validity of the data has some level of uncertainty (by its nature). The interpretation of the data too. I have yet to find an explanation (by Pickety or others) that made me understand the mechanics of the "Snowball Effect" at the centre of it all: 'r > g.' Once you get into the soup of morality and policy, well…
I like how he starts with the things he broadly agrees with. Even just agreeing that extremely uneven distribution is a problem and why, gives us a starting point. I personally take a slightly Marxist view on this. I don't think that extremely uneven distribution is politically stable, or compatible with democracy.
I am slightly doubtful of taxation as a solution. Taxation is stuck really. The problem is that most tax regimes are designed to maximize tax revenue while minimizing damage to the economy.
Consumption/sales taxes, income taxes and other middle class taxes are convenient in that they are very hard for people to avoid and they don't affect behavior much. marginal income tax of up to 60% is generally assumed to have a negligible effect on how much people work.
A 1% annual tax on wealth equates to a $10m annual cost of living in a country for a billionaire. Would they move (themselves and/or their wealth) to avoid it? Can some of that $10m be used to find ways of avoiding the rest of it?
I think that ultimately, wealth accumulation needs to change in order to change the structure of the economy.
Also, I like that Gates considers cultural norms, not just policy. What Gates & Buffet have committed to is a partial solution. If 20-30% of billionaires do this, it might be enough to change overall distribution somewhat.
In any case, more questions than answers.