Ugh, not sure what people aren't understanding about this. Bottom line is, this is an MMORPG. When you keep referencing single-player games in regards to challenge, you're killing your own argument because this is NOT a single-player game.

Yes, they should have content that appeals to both solo and party play; in regards to the main storyline, why not make it so that the early missions *can* be soloed (like those that need to be completed in order to unlock Grand Companies, obtain chocobos, etc.), but if you really want to advance the story and unlock higher-end areas, it damn well should require a group. This essentially separates access to content for casual and hardcore players.

Granted, not all of the later missions should require a group for every part of it, but like Jinko said, major milestone missions (like those that unlock access to said higher-end areas) should require groups, until access to means such as beastmaster/summoner companions and raised level caps are introduced.