Blue has 3 different teams, depending on which starter Pokémon the player choose at the beginning of the game. All 3 teams have the same average level, however.
The optional fight against Blue on Route 22 has been omitted.
The Fighting Dojo in Saffron City has been omitted.
Observations:
Wild Pokémon levels get rather erratic towards the end of the game. This is due to areas such as Route 22 being accessible very early in the story, and wild Pokémon levels accounting for this. Due to the way most Pokémon games are programmed, encounters stay the same across an entire area, with developers not able to adjust encouters for a specific patch of grass.
Wild Dugtrio can be encountered at a very low rate in Diglett's cave. These Dugtrio have a much higher level than the rest of the Pokémon in the area, and as such cause a large increase in the average level for the area.
While most Gym Leaders are higher levelled than the trainers in their Gym, Lt. Surge is slightly weaker than the trainers, due to him having a level 18 Pikachu in his party. The lowest level Pokémon that a trainer in the Gym has is a level 20 Voltorb used by the Rocker.
Compared to the original Red and Blue, levels in FireRed and LeafGreen tend to be slightly lower. Most notably, trainers in Gyms now have levels that align much closer to the surrounding areas, although late game Gyms do still have a small spike. The Elite Four and Champion battles are also at a slightly lower level.