So you were not searching for the wrong answers ;)
While customers might not have direct benefits from the improvements on the development side. You have to consider that This will allow you to decrease maintenance cost. E.g. think about a easier to use service console, easier to develop new feature, less workarounds for things that simply are not possible in older versions.
An other important thing you have to keep in mind is future upgrades. If the customer already knows that he is going to expand his architecture in the future than keeping his current applications up to date will also reduce the cost of bridging the version gap.
These however are all general things to keep in mind during an upgrade. More specific to Mendix 3 can be found in the release notes but the following might be interesting to your customer:
Progress: Long running processes (like deploying) can be cancelled more often and many of them use one popup showing the involved steps as opposed to a popup per step.
Memory usage: Thanks to a new file format both the Modeler and the Runtime use less memory when reading large projects.
Other improvements can be found in the appstore in the form of widgets that are not available in previous versions such as:
Checkbox selector Titler Empty grid label Collapsible panel Treeview
Just an idea... Next to finding more featurs, translating "features" into "benefits" that are valuable for your stakeholders could be more convincing too. Additionally, you may find more than 1 benefit per feature, and you will avoid boaring them with features that they are not intrested in.
eg "enabling and disabling of breakpoints without restarting" = faster building & faster troubleshooting when analysing support issues & incidents for business users.
Version control should be enough to convince them :) additionally, database synchronization can handle many more cases and timezone handling has been improved a lot. More can be found in the release notes.