A friend of mine wanted my thoughts on the Robin Hood Tax, so I thought I'd pass them along to you:
1. Though the page is vague about the issue, the tax would likely not be instituted by all countries evenly. Therefore, if this campaign has any success, you'll have some countries that incorporate it and some that don't. Financial transactions are *very* mobile-- as mobile as it gets-- and it would be very easy for those who would be taxed to simply move to a locale where the transaction would not be taxed. People want to avoid taxes and this one would be easy to avoid.
2. The tax seems to target "speculative" activity. Depending how you slice it, just about any financial transaction could be classified as speculative-- so there's an issue of exactly what you tax. Further, due to different financial instruments, you can generally replicate one financial transaction through a combination of others. So that could be another way to avoid the tax.
3. As always, there are deadweight loss issues concerning taxation, though some of the effects are described above. Though if you really want to split hairs, those effects described above are technically excess burden, not deadweight loss.
4. As always, the ability of governments to effectively use tax money to generate positive outcomes is dubious at best.
5. As always, there are political problems inherent in taxation in general.
Anything else? 2 would seem to be the short run outcome, 1 would be the long run.