Network capacity
Network devices (routers, switches, etc) have the capacity and QoS features required for voice traffic
LAN and WAN utilisation
Interaction with, and affect on, data applications
Effectiveness of network-optimisation tools to maximize utilization through compression and latency-reduction techniques

Disaster recovery
Duplication of major PBX nodes (dependin on your chosen VoIp solution)
Fallback to POTS (Plain Old Telephone System) at all or strategic locations
Protection against loss of power and power spikes
Emergency use mobile phones (which could be pay-as-you-go)
Emergency facilities with pre-provisioned network connections
Re-routing via the public internet

Security (also outlined as a seperate link)
Denial of Service (DOS). Attacks via a weakly protected VoIP element could flood the network disabling data applications, or conversely a DOS attack on a data application could inhibit voice traffic.
Hackers making free use of the telephone system, for example by fooled billing (making the call appear to terminate internally when, in fact, it continues externally).
Snooping on network traffic, soft phones and voicemail by various devices, or by call hijacking (routing a call to a different destination).

Standards
H.323 (Packet-based multimedia communications systems) issued by the ITU
SIP (Session Initiation Protocol) issued by the IETF

There are also other, proprietary protocols in use, for example Skype.

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