I use both Splunk and Cloudwatch dashboards on a regular basis. One feature that cloudwatch dashboards have which I really miss in Splunk is a shared time cursor across all charts on the dashboard. So that it's easy to say "What was happening in all these different metrics at time X?"
Something like the existing functionality for Charts under Format>Legend>Compare Series that would be active on multiple charts within a dashboard at the same time.
Now, I recognize that Splunk charts are more diverse and so this is probably a non-trivial feature, but it would be very helpful for my workflows. I often need to correlate phenomena from different systems or sourcetypes by time.
EDIT: To clarify, I mean that when I hover my cursor over a point on chart A which occurs at time X, I see a vertical line though the point. A tooltip shows the value of the point. Simultaneously, on other timecharts a vertical line appears at the same time (horizontal position) and a tooltip shows the value of the point on that chart at that time X.
I don't know what Cloudwatch is so maybe I'm not following but you can have the same variable timeframe for all charts on a dashboard by using a universal timepicker and setting the earliest/latest on every chart with the same token.