This is my question, but I guess I'll get the ball rolling.

I actually feel like I'm one of the few who didn't find this book to be completely mind-blowing. Piketty's Capital demonstrates that...

• In the developed world, inequality and the capital to GDP ratio were very high before WWI, fell dramatically between 1914 and 1945 as a result of physical destruction of property and changes in public policy, and have been increasing since the 1970's. Roughly all countries we view as developed have followed this trend.
  
• Inequality and the capital to GDP ratio were historically lower in the US than in continental Europe due to an abundance of land and a more rapidly growing population, but did not fall as much from the shocks to capital of WWI/WWII. Today, inequality is greatest in the US among developed nations.
  
• Capital as a share of GDP is defined by the long term savings rate divided by the long term growth rate.
• The capital share of national income is (by definition) the capital share of GDP multiplied by the rate of return on capital

And reaches the conclusions that...

• Capitalism's trend is toward more inequality indefinitely, and that the relatively egalitarian income distributions enjoyed by many developed nations after WWII were only temporary products of the shocks of war, and that modern economies will return to the levels of inequality experienced during what is known in France as La Belle Époque, meaning the years between 1871 and 1914. Piketty also claims that the increasing ratio of capital to national income will result in an increase in the relative importance of inherited wealth.
  
• A more wonkish version of the former point: that inequality rises when r, the rate of return on capital, exceeds g, the rate of growth of an economy (including demographic growth). In times of slower growth, r will fall less than g falls (i.e. the elasticity of substitution is more than one), leading to even greater accumulation of capital and increasing inequality. Aside from the last century, the rate of return on capital has always been greater than the growth rate, pictured by this graph:
  
  

• The equilibrium real rate of return on capital is 4-5%
  
• Progressive taxation, including higher taxes on unearned income and a global tax on capital, can help to alleviate this trend. He advocates global cooperation on corporate taxes, to avoid what in US politics has been called a "race to the bottom." I thought this was probably the most interesting part of the book, but its clearly done with less focus and precision than the parts on the dynamics of capital and inequality. Piketty claims an ideal top marginal income tax rate would be around 80%, and that it would not hurt growth, only ensure that benefits of growth were shared equally across income groups by discouraging disproportionate rewards accrued by the very wealthy.
  

He has some points about the value of financial regulation and on the history of progressive taxation, but not much else. The vast majority of the book, about 400 pages of it, is on tracking trends in wealth, income, and the capital:GDP ratio in France, other European nations, and the US. Piketty discusses 19th century wealth in the context of literature from the era, although he doesn't attempt to use this as a substitute for actual data. It has plenty of those graphs on what percent of income went to the top 10%, how much capital was held by the top 1%, etc. You'll be sick of those kinds of graphs by the time you finish this book, if you aren't already. Only about 80 pages at the end is dedicated to policy, and its almost exclusively about tax policy. I think maybe 3 pages are dedicated to the role of inflation in the accumulation and deterioration of capital, and there's virtually nothing on monetary policy.

So I was a bit unimpressed overall. I enjoyed the read but I think the hype is simply because Piketty has provided reputable validation for what people already think and believe: that inequality is increasing and it will continue to increase in the future. While this book provided a mechanism for explaining inequality, it spends a long time just establishing and evaluating the premise and nature of inequality and then suggests remedies that seem politically impossible to an observer on this side of the Atlantic.

Most people can probably just read whatever summaries their favorite economics writer churns out. This book is pretty heavy on statistics, almost to a fault. If this summary has piqued your interest, it may be worth checking out, but you'd be well served to temper your expectations from what the media has been saying ("most important economics book of the decade"). Its a good book, full of valuable, important and interesting information, but that probably won't change your view of the world. You'll either see it as justification for your existing beliefs or much ado about something you don't view as inherently bad (increasing inequality).