I consider railroading to be primarily two problems:
1) There is no choice, and not even an illusion of choice. The PCs are forced to follow the DM's story because there is nothing else to do, there's an uber-NPC making them do it, or they are somehow compelled. This is bad, but IMO, not as bad as...
2) The PCs have no impact on the world. The only actions that are effective are the ones that further the chosen story. Nothing else works. NPCs the DM didn't want killed will get away, alternative means of problem solving won't work, general cause and effect is ignored so that the plot will not be derailed.
Oftentimes the two come together. I don't see the presenting of one attractive choice and one boring choice as railroading. I don't mind being told, essentiallly, that "here is the adventure!" The important thing is for the characters to have some sense of choice, and that if they WERE to go down path 2, something would happen, and it would be at least slightly different than if they went down path one.
When I start a game, one of the handouts I give is called "Creating a Functioning Character." For the most part, it reminds people to be concious of the group when creating a character, but I think one line applies here as well: "There is a time when good role-playing needs to take a backseat to good gaming." Basically, it means that sometimes the most sensible path for the PCs, whether it relate to danger, adventure choices, interactions with the NPCs, or the way a character would react to something (the technical term being "But that's what my character would do!"), is not the most sensible choice for a GAME that is fun and fast-paced for everyone. Just like sometimes the rules need to be bent to provide a compelling, interally consistent world and plot, sometimes the believability of said world needs to take pause so that the game itself does not suffer.
As this applies to railroading, I think that the occasional nudge or bit of metagaming is necessary to keep the game moving. Otherwise the possibilities of the big wide world are too vast, and the PCs can wind up staring into the void going "...so what are we supposed to do?" The important thing is that the nudges be responsible: small, subtle, and infrequent. Even if they need to take path one, and path two will eventually lead to path one, the PCs need to be free to make the choices that they want, and know that something will happen as a result of the choice.