Druid Season's Herald does not seem to be applying caster bonus (and maybe spellpower bonus)... )=
How I tested this: While level 15 in fire elemental form (+1 caster bonus to fire) and in Druid of Summer season mode (+2 supposed caster bonus to fire) I cast Body of the Sun. In my combat log it says casting at lvl 16 ...this means I am only getting bonus from elemental form (+1)....it is supposed to be casting at lvl 18 (+3) to include the +2 bonus from Summer mode. This seems to be the same for other spells as well. This has also been confirmed by another druid in game.
Can any other season's heralds please test this and confirm. Maybe its just a display issue in combat log but I doubt it. If other fellow druids can test this and submit a bug perhaps we can get this fixed with the next patch.