Criticisms of the Nexus 7 revolve around a couple of port omissions
-- no HDMI output
Even though Google touts the Nexus 7 as a tool for media consumption, there is no way to attach the Nexus 7 to an HD tv. There is no HDMI output port, and no support for USB Mobile High Definition Link (MHL). MHL converts USB to HDMI signal.
I guess Google wants you to stream movies to your TV from Internet cloud storage, and use a dedicated home entertainment device such as the recently announced Google Q to connect the cloud storage to the HDMI tv. Or maybe this omission is due to the fact that neither the Amazon Kindle Fire nor Barnes and Noble Nook tablets have HDMI output.
-- no SD or micro SD card slot
The Nexus 7's built-in storage is limited to 8 GB or 16 GB. There is no slot for SD card. According to Anandtech, Android 4.1 has no support for attaching an external drive by USB. The Nook does have a micro SD card.
There will be much research by the hacker community into the Nexus 7 hardware and Android 4.1 software to rectify these two omissions.
Other features left out of the Nexus 7 include no removable battery, no active digitizer (thus no stylus input) and no second camera.