Not Just a Gateway - WSO2 API Manager is more than an API gateway. It is a complete platform for API Management, which includes an API gateway, security, developer portal, publisher portal, API lifecycle management, analytics and much more. It has a very comprehensive feature list and you can read about the feature list here.
Cloud and On-premise Options - WSO2 API Manager is available has a public cloud offering from here. Or it can be downloaded and installed on premise with different deployment options. Your organization can take an iterative approach and start small and grow into a API centric organization in an iterative manner.
Flexible Deployment - This is one of the strongest reasons to go for WSO2 API Manager. It offers flexible deployment patters as each organization has different network, governance policies. You can start WSO2 API Manager in different modes (profiles) to act as a gateway, developer portal and publisher portal. You can deploy different profiles at different network zones (eg. DMZ) adhering to organization network and deployment policies.
Part of a Platform - As all WSO2 products, the API Manager is built on it's comprehensive platform. That means whenever additional functionalities are required one can extend into those areas such as security, integration, real-time analytics and mobile device management and they would work together seamlessly with near zero effort. So what if you don't want any more WSO2 products? No problem, just jump into the next point.
Openness, Modularity Extensibility - WSO2 API Manager is 100% open source and distributed under Apache License 2.0. It is built on open standards such as OAuth 2.0 and has all of it's functionalities available as APIs. It is modular and extensible allowing to plug into external Identity Providers. All of these leads to into zero vendor locking.