The users will not be able to view their address book and chat list, unable to make call with WhatsApp, nor able to accept chat notification. This will make WhatsApp a completely useless application.
While WhatsApp forces its way to make users accept the terms, there were parties who stood up and claim that WhatsApp had violated the regulations of Apple AppStore.
According to Article 3.2.2(vi) of the Apple AppStore Regulations:
WhatsApp currently had required its users to accept the unfavourable terms unconditionally, else they will limit the user’s access to the app. This is clearly a violation of the regulation and the the developer will risk its app being removed from App Store and being banned from running other apps too.
However, Apple and WhatsApp have yet to respond to this matter at the time of writing.