What would be required to prove your identity? Would you be able to tweet anonymously, or must you tweet under that identity? There are some issues with that, for example people without government identification would not be allowed to tweet. Perhaps you could use unique fingerprints, but that turns into a huge privacy concern and I can never see that being accepted. Maybe there are some unique bio-markers that could be used which people feel would be irrelevant or otherwise useless enough to not be an invasion of privacy.