Does it keep the basic parameters of class abilities? Magic-users don't get to use armor, clerics are restricted on weapon choices, etc.?
Yes … to the very limited extent I understand such things. Weapon & armor stuff is handled, basically, with feat-type stuff, just like in 3e and 4e. If you're a Wizard-guy, by default you have access to a very few weapons, and no armor training. With a little bit of effort (multi-classing, grabbing a feat after several levels, or being a dwarf) you might solve some of those problems if you wanted to, but it's at an opportunity cost of being a wizardly Wizard who wizards.
Thieves Rogues are still backstabbing skill-monkeys, and Clerics are slightly more individualized based on their deity's portfolio. Of the basic four classes, the biggest change is the Fighter. (I kind of think each edition of D&D should be called, "What Have They Done to Sweeten the Fighter Now?") Fighters don't get an appreciable attack bonus (THAC0 / BAB / to-hit chart) over the other classes. Instead, they get extra actions during a fight, and become gradually capable of almost superhuman feats of Strength, or Dexterity, or Constitution, or what-have-you (and killin' stuff is just one portion of that).
It actually looks pretty playable; it's appreciably simpler than 1e, but maybe a touch more complicated than B/X. But we're really seeing only one-third of the full design: the Monster Manual isn't out yet, and the DMG is pushed back until, I think, early December. Which could mean that WOTC is over-worked, or that the design is tricky, or just that they've had various normal delays. (Jamie Malizewski done stole their money, maybe.)
