The ability to get up an running quickly without having all my eggs in 1 basket (we used both Tomcat as well). I like how easy it was to add new applications or services using other frameworks such as Spring Boot which allows us more flexibility when developing our systems/services than just pure java servlets would allow by itself. Not much really but some minor things - we had issues getting tomcats SSL certificate working with resin so i think there were workarounds required at times but nothing majorly annoying. We have been able to deploy fairly small scale apps very easily compared to what others are doing nowdays even though they use JEE containers!