I've been looking into the whole MMO thing, and from what I can tell you won't be able to see "everything" in the game if you play offline. However, you can play the game offline. Now, what "everything" entails could be rather damaging depending on the quality of the content, but it's not like the game suddenly throws up a wall after a few hours and says "sorry, but you can't play any more unless you pay a subscription fee." That said, Dragon Quest already had one strike against it with me for being a series that steadfastly refused to evolve past the 1980s, and making such a large part of the game more or less an MMO with a subscription fee (which has yet to be determined) pretty much killed any remaining interest I might have had in the game. There are plenty of good singleplayer-focused RPGs out there without having to waste time on a game that will try (and likely fail) to straddle the lines between single and multiplayer content.
I'm actually struggling to think of a game (outside DQ IX and Monster Hunter Tri...I guess...I never played those and never want to) where trying to merge single and multiplayer actually turned out to be a good thing. Square-Enix already spectacularly failed at that this year with Mindjack, as did another company with Brink. Why can't singleplayer games be strong singleplayer experiences, and multiplayer games be strong multiplayer experiences? Oh right...because Square-Enix lost a fortune on Final Fantasy 14, and now they're scrounging for any way to extort cash from their customers to make up for that.