We (no matter the country) could save billions in administrative costs simply by moving all tax to point of sale style VATs. (As a sidenote this does not mean that something like the FairTax proposal is a good example of this concept.)
For much of the lower/middle class, a significant, sometimes 100% of their income goes to consumption. For the wealthy, consumption as percent of income can be very small.
The current system is regressive in practice, though not in theory. If this wasn't the case, Buffet wouldn't be paying 17%.
Spending 100%+ of your income on consumption can be the norm for lower income earners, it is true. That does not extend however into 100%+ of income spent on consumption that is required for basic sustainability, especially in rich countries such as in North America or Europe.
There is no incentive under the current system for any income bracket to spend within their own means, regardless of what those means may be. The high levels of unsustainable personal debt illustrate this perfectly.
This results in overspending on "luxury" (i.e. not "required") goods - even by the "poor". There are few incentives that encourage the poor to establish wealth through savings. There is also no incentive in the system to have individuals increase their income to match their spending, although that's admittedly a much harder accomplishment.
With an income tax system that isn't perfectly flat, the problems you highlight are simply hidden, if not encouraged. With a flat based taxed, they are not, but it is regressive.
With a flat consumption based tax with a level of social control through either personal spending exemptions or through item based exemptions (baby food, for example) we could do a better job of managing the regressive nature of tax systems in general than we do right now.
In addition, we expose basic economics to those who have difficulty understanding the concept by providing strong incentives to build wealth and consume less, regardless of income bracket. This is something we should do.
FairTax, in my personal opinion isn't the best example of where to draw these lines. It is rightly criticized for that, but that says nothing about the fact that overall, taxing consumption is a better idea than taxing income - a move that was largely bred from the need to fund a massive increase in government spending.
A tax that allows for funding of government programs, encourages the creation of wealth and the growth of personal income and discourages rampant consumerism (as in buying things you can't afford - and having prices bubble accordingly) is a much "fairer" system in my books.
I discount a lot of the notions of this type of system being inherently regressive because that argument insinuates that the current system is not. I believe Mr. Buffet quite aptly supported this assertion with real numbers a few years back.