While I generally abhor taxes, I really like the idea of a financial transaction tax. It would effectively eliminate the profit margin for high frequency trading and remove a LOT of volatility from the markets. IMHO the market volatility is causing the average person to doubt the stability of the economy and hurting the entire ecosystem. Even a small transaction tax... Say, $1/EUR per trade would remove the incentive…
By drastically reducing volume it would also reduce liquidity and increase spreads. Sweden tested this in the eighties and abandoned after seeing the market substantially dry up (and the resulting taxes as well).
There are lessons to be learned from Sweden's experience, but I'm not sure you can necessarily point to as a definitive case study and say "See? It won't work."