It's not obvious enough yet when the code is ready to cast, is casting, or has been cast. Beginners don't realize what's going on. Here are what things look like now:
Any improved design might think about improving more than just the cast button, like changing the look of the spell editor itself. And anything involving the cast button might consider a different button design, because this one falls over for i18n:
What I'd imagine we'd do is to come up with a new design for the cast button that gives it more space, more visual prominence, and a much more obvious animation. It would be good if it looked cool, too! See the Tome docs for info on how this stuff works.
Note that the cast button does include this kind of world-casting-progress fill as it's casting. That should either be preserved in any upgraded design or moved to some other area of the interface.
Having moved this down to its own area in the spell view, it's looking much more prominent now.