Still skeptical about the gunplay, Destiny has that perfected.
The traversal and mobility look great, and the verticality element is awesome.
Lastly, maybe I've missed information, but I'm still not really sure what's going on with class/javelin loadouts and abilities. Is it all through gear, or are there different ability subtypes that you choose from like Destiny that are baseline and not tied to gear, or both?
OK so it'd be easier if you start comparing it to Warframe rather than Destiny but since msot people are familiar with Destiny, I'll explain from that perspective.
First of all, no "ability" is shared between javelin. The things that are shared between javelins will be gear and even then, certain weapons or components will be javelin specific. For example, the real heavy duty weapons can only be used by the Colossus and so forth.
Second, each Javelin has three abilities active to you that you choose from. One is a support ability, the second is usually an AoE ability and the third can be Javelin specific. Each "category" has a list and you choose which abilities you want. See here to get a better idea:
https://vulkk.com/2018/09/02/all-anthem-javelin-classes-explained-abilities-and-roles/
The subtypes of abilities that you mention can most definitely be achieved with gear though its not quite subtype necessarily than it is specialization. They showed a build on Stream of a Ranger with a focus on hovering, so when he used his missile launcher in the air and got a multikill, he could do a grenade ability which would have 50% more damage for a limited window working perfectly as a combo. And while he was hovering, his cooldowns were reduced by 50% so its a combo that he could perpetually "spam" by utilizing air superiority.
For a better explanation of how the gear system is EXACTLY going to work, watch this video:
Have they said in any of the videos if you can inflict damage to enemies by flying into them?
I don't think so but each javelin has a unique melee ability (with the interceptor "specializing" in it). You should generally be able to close the gap to an enemy fairly quickly. Plus there's close range abilities and weapons like the shotgun.