I like that Ember's tagline is about ambition.
It's easy to write an empty, temporary tagline. "Simple and good". "Only 4 kilobytes." It's harder to write a tagline that generates principles which stand the test of time. "Simple and fast" are in opposition. "Only 4 kilobytes" is unlikely to last if the software evolves.
"Ambitious" as a tagline speaks to a number of qualities, but none of them are in conflict. An ambitious project could choose the boring tactics in favor of rapid delivery. Another ambitious project could aim for longevity. Yet another ambitious project might aim to have a large impact in the world or aim to do something previously considered implausible.
For a framework the size of Ember, that's an appropriate diversity of goals. But they're all covered by ambition. The tagline unifies and clarifies as much as it inspires.
