Here are my predictions how it will shake out. Let's revisit it in a year and see how I did.
- Overall Design Philosophy: Modular, rather than exception-based. Lots of watertight compartments you can screw with and not sink the whole ship. Emphasis will be on imagination, open-ended, interpretive play, less on mechanics.
- Armor Class: They will use the high = good, low = bad AC scale of 3e and 4e.
- Saving Throws: Will follow the 4e model of being treated as additional forms of defense/armor, but will inexplicably be called saving throws anyway because the retro aspects of 5e will mostly be superficial nods to earlier game elements.
- Generating Ability Scores: multiple methods will be described, from strict roll-in-order to point build. The preferred method will be roll 4d6, discard low die, arrange as desired.
- Ability Scores' Effect on Play: More robust than in 1e, with anything above an 11 providing at least some benefit, but the list of bennies will be short. Strength will add damage, Con will add HP, Dex will add AC and missile accuracy, Int and Wis will affect arcane and divine spellcasting, and Cha will affect NPC interactions.
- Classes: The basic game will include only wizard, fighter, cleric, and thief (yes, thief, not rogue, another easy cookie to toss at the retro crowd). Bards, barbarians, druids, and rangers will be add-ons. Weirdly, class abilities will get fairly modular again, with separate mechanics for turning undead, picking locks, and so on.
- Races: The classic elf, dwarf, human, half-elf, and halfling. Half-orcs and gnomes will be optional. Dragonborn, eladrin, and tiefling will not be standard issue. Races will get some ability score tweaks and a paragraph or two of fluff in the basic game and that's about it.
- Skills: the skill challenges of 4e will be quietly throttled with a silken cord and replaced with a short list of proficiencies, carefully limited to those most likely to be used in an adventure. "Roleplay it out" will be the preferred problem-solving method.
- Combat: Far fewer conditions. Marked and blooded will be part of a tactical add-on. Daily and encounter powers will go bye-bye. Ditto healing surges. Combats will be shorter and deadlier for PCs.
- Spells: Out-of-combat spells will come back in a big way. Rituals go bye-bye. Several game-busting spells like scry, fly, and whatever will disappear or become far rarer.
- Magic Items: Won't be gimmes -- you'll find them in the course of play. Look for a general powering down of magic items. A +2 sword will be pretty cherry.
All in all, it's going to be a really stripped-down beast, if compatibility is a serious goal. The OSR Greek chorus is going to be very much on the mind of the WotC team as they go forward.