Consistency - Fabulously Optimized must be sustainable, 100% server-compatible and it should work similarly across versions. Every new mod and change is carefully considered to make sure it would work well, feel vanilla and be available in the future.
Familiarity - main priority is having parity with OptiFine, but sometimes Forge, Bedrock Edition and snapshots are considered as well. Note that the parity does not mean identical behavior, existing FO users and vanilla-like Java Edition users' preferences are highly considered in general.
Neutrality - the modpack should only give you the essential tools and fixes to customize your game, not include any opinionated extras itself (e.g. a mod that adds a minimap, armor HUD or more sounds).
Privacy - you shall not be punished for "having too many mods" or "using the wrong zoom mod". In simpler terms, the client tells as little about itself to the servers, mods and Mojang as possible, even though the mods are already allowed by most servers' rules.
Updates - Fabulously Optimized receives fast updates and is one of the first modpacks to upgrade to newer Minecraft versions. That's why its mods should also be consistently supported to not slow down the modpack's development.