Of course, we could just tell you how all of this fits together... but why would we do that when we can just show you? So click on past the break for the video itself, and then keep reading for more specific details on these new mechanics. Even familiar things like cast bars are a little different here, and you'll want to see some of this stuff in action.
It's important to note right off that for all of the novel stuff shown in the video, some things still aren't revealed yet. A few traditional tab-target abilities do still exist in the game --generally reserved as heals that weren't as much fun when changed over to other mechanics. So if you really prefer that style of play, you won't be locked out of the game (although you might want to play a healer).
The biggest thing that all of these abilities mechanics do from a design standpoint is allow players to have access to a much wider range of abilities with different mechanics without trying to force everything to work the same way. Charges used for individual abilities allow the designers to create more intricate mechanics for those abilities, rather than relying on other talents or abilities to create synergy. It also means that abilities can be more finely tuned for the class as a whole rather than adhering to overall patterns; some hold abilities can be maintained indefinitely, others only as long as you have certain resources, still others only for a limited time. The mechanics are the same, but the details can be tuned by development to make sure a given ability fits for a class.
For new players, it's a bit much to jump in to, but the entirety doesn't come right out the front gate. Both telegraphs and ability complexity are slowly increased over time, with the earliest abilities being far easier to understand at a glance. Players will be able to use those early abilities for a while, at that; ability loadouts will be changeable based on both situation and individual playstyle. Players will also have an equipment slot for a special item to add an additional ability to the overall rotation, creating another point of variability.
So what abilities are you forced to use? At the moment, none. While the team is still working on the best means for allowing player specialization, the current system being developed allows for direct improvement of individual abilities along with a separate system for more general improvements. So if you really enjoy hold abilities or rapid-tap attacks, you can load yourself up on those and enhance them to be the heart of your arsenal.
Not that every class will have the same spread of abilities. While each class has a range, there's a different tempo for each individual class and its abilities as a whole. This is true for tanking, healing, and DPS. Espers and Spellslingers, for example, both can be healers, but Spellslingers have more abilities that serve as straight-line heals or help a single target. Espers focus more on area heals over time. Melee DPS has a larger array of instant abilities and skills with less lead time, along with tools to close distance easily, while ranged DPS has more ornate cast times in exchange for... well, range.
Combat in WildStar isn't meant to be just a matter of tapping abilities and hoping for the best. There's meant to be a lot of careful decisions involved in the moment-to-moment gameplay. All the mechanics are meant to keep you on your toes and mobile, and if that sounds like your cup of tea, you probably can't wait to see what happens when the team stops talking generalities and starts talking in specifics.