How much railroading is necessary?

Here's a question. Are pure sandboxes satisfying? Is here is a world, there's some stuff going on enough? Or does there need to be something more? I think so. In the average campaign, there is the expectation that the character are adventures. In fact, the world is usually laid out with the assumption that the characters are adventures. Otherwise, why would the GM bother detailing adventure sites?

The question I'm considering is to what extend is setting up the world with certain expectations of the players railroading.

The GM has only so much time to invest in gaming, and thus has to prioritize what sorts of things to flesh out over others. Choosing to flesh out cities and societies will create a world a different feel and expectations than one in which the GM has decided to flesh out an enormous megadungeon (Vats of Mazarin) or a world that has multiple smaller dungeons spread throughout the country side (Agrivaina). Likewise, concentrating equally on on multiple aspects creates plethora more outcomes.

To some extent, the GM has to make some basic decision as to where the fun is. Yes, the GM can improvise and wing a game, a great satisfying game that has nothing to do with what has been previously fleshed out. But there is typically the expectation that the GM will let the players know where the fun can be found.

[To be clear, I'm not saying this is a problem. Not at all, it's just interesting to me. Many of the players in my home game desire more direction and a more epic feel than most G+ players, so I have to set up worlds and adventures differently.]

When setting up Agrivaina, I made the decision that the world would not push the characters in a particular direction, instead I created a list of adventurers are various type. There are 2 megadungeons. A few micro dungeons. Some wilderness situations. A seas adventure. And some intrigue. None have any particular consequences that will negatively effect the PCs if left untouched. However, I do have a limited number of hooks - a limited number of sites I can let the players know that there is fun there (e.i. the ones I've thought of). They can always create their own schemes, but I still feel I'm railroading them towards ideas I think are cool (as opposed to ideas I think are uncool).

My preferences still have a dramatic influence on the game's direction. I'm railroading the game in the direction of adventures I think are cool. If I don't think it's a detail interesting, I'll probably say something else - even when improvising and running off the cuff.

At the same time, I don't feel like I'm doing it wrong. Quite the opposite. Often times a blank slate is a dull slate. It's always more interesting to have a some ongoings in the world to riff off of.