You can read about it for yourself in the beta forums...
Relative to RMSS/RMFRP, the skill system is much simplified. Skills are organized into categories but you only purchase skills (not categories), and all skills have the same progression. There are also fewer of them. There are rules for using similar skills which give some of the same benefit as RMSS's categories. Professional bonuses are mostly per level rather than up-front. Moving maneuvers and resistance rolls are streamlined. The number of armor types has been reduced to 10 (not 20) but there are new rules for partial armor and called shots which allow treatment of more combinations. The assumption that heavier armor makes it harder to dodge has been removed from the tables so that armor is clearly advantageous (not always true in RMSS). Combat rounds have been shortened to 5 seconds (which among other things makes movement during combat a little more tactical).
The rules also include systems for creating your own races, cultures, and professions.
The changes in Spell Law are not dramatic, although there are changes to some spells reflecting how the combat round has changed (e.g. haste). Also, healing spells now work in the framework of the recovery rules (i.e. they give a substantial bonus on the recovery roll and may also reduce the result by a significant factor, rather than being entirely separate). One notable change is that there are no blanks in the spell lists -- every spell list has a spell at every level.
The release date is "when it's ready." Arms and Character Law will be first, it is moving towards a final draft now which will inform work on the other books. I cannot guess how long this will actually take. It will also need layout and art of course. Spell Law will be next, it probably needs less work than A&CL did to get to the finish line (e.g. because we did some big organizational rearranging in A&CL, which is not needed in SL). Creature Law and Treasure Law have only been through one open beta draft (whereas the others have been through two) and will need some adjustment based on what happens in A&CL, so they will take longer.