Besides being unable to make traditional voice calls, one difference is there is no earpiece, so unless you have headphones, your VoIP calls, video chats, etc... will all be on speaker. Since it is a tablet, it's obvious why you wouldn't have one and it would look crazy holding that giant thing up to your ear. Another difference that people also tend to overlook is SMS. Since there is no phone number associated with the tablet you won't get SMS service. That can be mitigated by using services such as Google Voice which uses data instead of the SMS protocol to send these messages but yet are delivered to the recipient as SMS messages.
Due to the above you'll be missing the phone and messaging app components of the android OS on the nexus 7 (although as of 4.4 messaging app has been integrated with hangouts). Also you'll have support for multiple user profiles on your tablet, which is not an option on android phones.
BTW, you'll probably need to wear cargo pants to accommodate a Nexus 7, as it would probably be too large in your front pocket for you to sit comfortably.