One reason "professional grade" is hard to define is because it's going to get manufacturer's in trouble by setting unreasonable expectations. It's easy, for example, to say that a metal bodied lens is more durable than a plastic composite... but that's not strictly true. The metal body will show dings; the plastic composite won't. The metal body will transmit vibrations easily; the plastic composite will dampen them. Fiber-reinforced plastic composites are often chosen over metals for various construction now, too. Is metal really better?

I hope a minor mention of moisture sealing is not tantamount to a lens being weather sealed.

This is one that always gets me: what is your expectation of what "weather sealing" should provide? IP51 (dust protection and water drips)? IP65 (dust tight and water jets)? You need only refer to the IP code to see that these are actually somewhat vague about the level of protection, and figuring out how to classify the environment in which you're working is an essential part of understanding what your requirements are.

Stepping up to IP67 or 68 for typical waterproofness is going to be extremely difficult. Think about a lens extending for focus or zoom and the need for full-on sealing in that area. Don't forget the lens mount, too... and you need a camera to meet those same requirements, too. These requirements are what gave way to systems like the Nikonos and Nikon 1 AW1.

What are the features beyond focal length and aperture that makes a lens suited for profssional use with higher price tags?

Why are you looking beyond focal length and aperture? There's some overlap in current lens focal length options, such as the 18-55 and 17-55, 80-200 and 70-200 (and another 70-200), for example. Amongst these options, aperture is the thing that separates them.

There is nothing to consider further. If you need a medium-range telephoto zoom lens you can choose from the three options I noted above. If weight is the priority, you choose the 80-200 and accept the slow aperture. If a fast aperture is the priority you choose the 70-200 2.8. There's no consideration for buying a 70-200 2.8 "basic" or 70-200 2.8 "weather sealed." That's not an option. There's no reason to consider it.

If you're looking at the 16-35 f4 because you want a wide angle zoom then there are a few other options available, to you, too: 17-35 f2.8 (older, faster aperture, though not quite as wide) and 18-35 f3.5-4.5 (even less wide, variable aperture, and slower at the long end). The 17-35 is a previous generation top-of-the line option. The 16-35 and 18-35 are both current, and of them the 16-35 is clearly the "professional" option with Nano crystal coat, heavier weight, and plenty of reviews to show it's optically better. There is no "more professional" option, and because nothing exists there's no need to define it.