Though believing Amazon's core retail ops are performing well - ChannelAdvisor's June data was quite healthy - Citi notes many investors have grown frustrated over a lack of earnings leverage. It also thinks heavy capex and major price cuts for AWS will pressure margins over the near-term.
Also: UBS is out with a cautious note, stating survey data indicates heavy opex will limit margin upside.
Shares have underperformed the Nasdaq this year, but have also staged a healthy rally from their May lows. Its 2015 EBITDA estimates are below consensus.
Update (12:10 PM): Shares have shrugged off the downgrade, and are now up 0.4%.