I have a suggestion.
Every time we change a property of an element we have to hit OK to see the result. If we want to correct it, we have to reopen the properties windows. That's a lot of extra clicking, especially when we are about to make some creative decisions. Not always a quick preview in the properties window is enough to surely tell whether we like the outcome or not.
So my suggestion is to introduce an "APPLY" button in every possible element's properties window, which will update the change we made to the model but won't close the properties window, so we can see the change instantly and keep tweaking properties.