Just a couple of ideas I've had.
It should be possible to reset the simulation timer as well as add a custom m/d/y.
Parent body and distance to parent body should be in the left-click infobox of an object. It is important to know clearly and easily the distance your planet is from its host star. The orbital period would also be an interesting information (see 1st attach.)
More compact and transparent infobox. It blocks too much of your view of the simulation. Is it really necessary to have a whole rectangle to display a small miniature of a round object? (see 1st attach.)
The "stars" tab should come before "planets" in the add menu. When you create your own solar system, you add the star first. There should be an order. First the star, then the planet, then the moon, etc. (see 2nd attach.)
Add the Andromeda and the Milky Way galaxy please. They are already rendered in the "galaxy collision" simulation. Just let us be able to use them in the next version, please.
Turn grid on by default. It looks modern and gives the player a better sense of position, movement and scale. The zoom ins-outs are
logarithmic (base 10), and it might confuse some players. They will think the universe is much smaller than it really is.
Radius should change instantly when the mass of an object is changed. Really, if I change the mass instantly, why does the radius have to change slowly?
more on-screen text for a better communication with the player. It can be any sort of message. I feel kinda lonely playing this.
That's it... I'll think of something else later on