ok, I'm a fair bit into the pre-history section so here's a few of my thoughts so far...
I'm really digging the combat in this. It takes the ATB (Active Time Battle) system and adds the strategic element of a grid based system. As battle begins, you can move one square in any direction you wish. As you move a space, all time bars (which is called a Charge Gauge) for both player characters and enemies slowly fills. If you haven't made it to an enemy in time to act, there's a very good chance they will before you can. It really adds a but of forethought into movement.
Some skills have a specific area they will affect. The one skill my character has currently, as an example, is a jump attack that requires you to position yourself further away from the enemy in order to hit. Others encompass a huge amount of the battle ground that can hit multiple enemies, but be warned. Some skills leave an area of effect on the ground that can damage your other characters if they are currently occupying those areas.
As far as I can tell, there doesn't seem to be any random encounters thus far. The main character in the pre-history section can "sniff" the air to "catch the scent" of creatures or items scattered around the area. These seem to be limited but you can refresh the area by going in and out of other locations.
So far, so good.