The UI is a huge pain point with our project managers. A simple action like dragging and dropping tasks, becomes cumbersome because if they hover over the new location just a second too long, instead of the task just being moved, it automatically makes everything underneath it a child task. This messes up all of their dates, etc. which results in a lot of manual effort to "fix" it back to the way it was. In-line edit functionality is great for somethings but project/schedule management is not one of them.
The other thing lacking that our PMs have problems with is the fact that when they are working on their project schedules, notifications, etc. are going out to team members and they haven't really had a chance to view the impact of the change they are making to the overall schedule. As a result, they could potentially send out numerous emails to team members until they get the schedule adjusted just right. Tool really needs a publish feature. The legacy gantt addresses some of this, but it is not ideal. A work-around was suggested by Workfront to alter the project status temporarily from in-flight to planning to suppress emails, however, this creates other issues if people are actively trying to run reports or if a resource is on the My Work tab and their tasks all of the sudden display a message that they no longer need to work on it. A better solution would be to have another tab that looks like the Tasks tab, but is actually a scratch pad for PMs to apply schedule maintenance and do some modeling to verify the impact of the changes to the overall schedule, before they actually apply the changes. Once they've verified their changes, have a "publish" button on the tab that would commit all of the updates to the DB and display on the regular Task tab.
Another one, PMs are tired of random forms attaching to their tasks. We have some old forms that are no longer used, however, because they contain a custom field that we ask them to populate on a standard task view that we have, Workfront takes the very 1st form ever created with that specific field on it and attaches it to the task. This also results in a lot of unnecessary maintenance. It would be nice it you could specify on the project the default custom form(s) that should be attached to a new Task or Issue that is added to a project.
Finally, PMs are tired of people logging time to tasks that have been marked complete. They have an extra report to monitor and then have to go back and follow-up with each person to get it corrected. It would be nice if Workfront would not allow users to log time to tasks that have been completed or better yet, not log time to tasks that have been completed beyond the Actual Completion Date.
Sorry, I know that on the surface some of the above may go beyond a basic "undo", but all of them result in additional maintenance that our PMs just don't have the time to do. So technically, they are "undoing" something whether it is changing the status to do maintenance, moving a task, dealing with the wrong custom form, or having time entry corrected...in the grand scheme of things they are all "undos".
I've opened ideas for most of these on this exchange, but it seems that its easier for people to open a duplicate suggestion rather than searching existing ones and voting them up or down. :)