Board Game Arena

Maybe I am missing something, but there seems to be no incentive to act first and no penalty for passing until opponent acts first?

first player has 6 manas, and then both 9 manas, who summon first has the oportunity to attack first (if the creature dont have the hability)

So if a first player passes, it will put him in a more favorable tactical position, because he then will be able to react to opponent decisions with full 9 mana (considering that no creatures can attack a bastion/fort on a summoning turn, only other creatures in case of "rage")?

The short answer is tempo.

Going first gives you less options but gives you tempo. The game is about getting creatures on the board and making them stick. Sure going first doesn't give you anything to react to, but creating a problem is just as good as solving one.