Coinbase took a step yesterday towards lowering the friction of buying and selling bitcoin in the US.  Customers that live in a small number of states can now hold a USD balance in their Coinbase account.  This allows instant buying and selling of bitcoin instead of Coinbase's current week-long process.

This could help alleviate one of the biggest complaints about Coinbase.  There are many reports of transactions being cancelled for "suspicious activity" days after they were initiated when the price moves significantly.  It is difficult to tell whether these reports are simply a case of selection bias, or if there is a true correlation.  Either way, instant sales should cut down on this problem dramatically.

Coinbase has allowed access from 16 states.  Most of these states are known for having an easy process for obtaining a Money Services Business license.  This may have driven availability, or it may have been a desire to ease into this new feature and give themselves time to work on fraud, reliability, and scalability concerns.

Coinbase is still not a true bitcoin exchange.  They are the counterparty in all trades, and you can not enter limit orders, multiple orders, or implement the myriad strategies bitcoin day traders employ.  However they continue to make progress in providing a reliable way for americans to purchase bitcoin.