Notice though that all races get bonuses to one mental stat and one physical stat. Giving Half-Orcs STR and CON would beg the question of why you'd ever play one as anything other then a tank.
I Like the idea of Druid being a favored class for Half-Orc, It does also fit very well with the idea of a more primitive race and still benefits from the bonus to WIS.
Classes that don't have favored options (not counting Human and Half-Elf) currently: Druid, Paladin, Monk.
The stat bonus on the Halfling doesn't lend itself to Monk but the description of the race does very much so. If Dwarves were given Paladin instead of Cleric, Half-Orcs Druid over Cleric, and Halfings Wis instead of Int and Monk instead of Wizard. These three classes would now be represented. Cleric would now be left in a lurch though.