Test Test Test.
Before you add the data center ready xml param in the atlassian-plugin.xml - make sure to test your app. Depending on the functionality of your app, you may need to to test things like cache invalidation(or lack of validation across nodes), data base load (what happens when multiple nodes are messing with the database at the same time), event propogation (do you need every node to get the event or is it ok just to be local).
Once you have that - then think data volume - data center instances are more than likely the ones that will have the “interesting” configuration. If you’re showing a list of users - how does that handle 1k users or 10k users or more… It might not be a performance issue but rather a volume issue.
Now, none of this is data center specific stuff necessarily - just things that you’ll be more likely to encounter in a data center env (well with the exception of the intra-node communication issues). Your app may work in a datacenter env out of the box, but don’t assume that.
Read over https://developer.atlassian.com/platform/marketplace/guidelines-for-data-center-app-development/ about some of the concerns you’ll need to take into account.