The first thing that you would notice about this tool is the speed at which the tests are executed. Based on my experience with other functional web testing tools like Selenium WebDriver, IBM RFT, Telerik, Sahi etc., I found WebDriverIO to be at least 5X faster in comparison.
WebDriverIO comes with a built-in test runner wdio, which can test across multiple browsers in parallel. The maximum number of parallel instances that should be launched during execution can be defined. This is a great feature, which helps you do some form of concurrent testing, in addition to functional testing, as you can launch tests on multiple browser sessions. Testers can run different specs or user stories in multiple browser sessions, which actually simulates usage of multiple users in real life trying to perform different operations on the application concurrently.
WebDriverIO was designed with the Page Object Pattern in mind. It provides inheritance between page objects. You can abstract all page information from the actual tests.
Test Results can be reported in Junit XML format or Allure Report format. Allure reports are packaged with very user friendly test reports in HTML format.
WebDriverIO provides integration with CI tool like Jenkins. As it is widely known, Jenkins provide detailed test results with history charts, stack-trace information of your test execution.