I would say it's a 3-way tie between how easy it is to build and update custom forms, updating project timelines and building reports/dashboards. Before my team started using WorkFront, we had to use 3 different systems to get a single project started because there was no central place to house all the information and capture what the details were for an update to our website. Since moving to WF in 2014, i cannot think of using another system.
As a project coordinator i'm responsible for reviewing project request submissions, then converting them into projects and building timelines around all the deliverables and due dates. I can honestly say the system is easy to learn and intuitive to use. I can build timelines in mere minutes, assign out tasks to team members, build a report or dashboard of project requests stats for my manager, post an update on where the project stands, etc. The list goes on and on. It takes a universal design seen a lot on social media platforms and uses it as a skeleton for how easy the system is to learn and use. Most changes or updates can be made in seconds and helps keep everyone organized with their plate of work.
I heavily dislike how WF as a company seems to spend their time updating parts of the site that my team does not use on a regular basis, or on things that nobody was asking for. Case in point, they just released a system update to include emojis in the update section of a project. This is an unnecessary update that nobody was clamoring for, and when it was pushed live it broke other things on the site my team uses on a regular basis. So as a company they need to focus on making updates to more important things that need to be worked on.
i would recommend lots and lots of training before throwing your users into the deep end of the pool to use the system. It's easy to learn and very straightforward, but i will admit it takes a bit to get use to how everything works. So take as much time using and practicing using the system before implementing it throughout your organization.
As I mentioned before, before my team started using WorkFront, we had to use 3 different systems to get a single project started because there was no central place to house all the information and capture what the details were for an update to our website. Now we have a central intake system that houses all our requests, and makes it easy to review, convert and assign our to the appropriate team members to make the updates and push live to our site.