My working assumption when reading the previews is that the game works something like the following.
Each class has at a given level the same number of quasi-vancian abilities, separated into 'at will', 'per encounter', and 'per day'. (And possibly further separated into 'silos'.) At 1st level you might have 2 at will, 1 per encounter, and 1 per day ability. This will slowly increase to say 6 at will, 5 per encounter, and 3 per day abilities at 30th level - plus things you may get from feats, your race, and possibly being trained in skills. (These numbers are of course purely pulled out of the air, and merely markers for how much flexibility I'd expect you to gain at higher levels. They could be less, you might not ever increase beyond 2/1/1, but I seriously doubt they will be higher.) Once you acquire an ability it is yours to keep, but even though you know an ability you won't necessarily be able to use it unless you've chosen it to fill one of your available slots.
One thing I'm not sure on and which I haven't seen hinted at is how often you can swap out your selected powers. Can you only do it when you level up? Can you do it while resting between encounters? Can you select a new kit of powers each day? Each pattern has different merits to it.