>>45850982By railroading, he means certain optional events and open sequences from the original games must now be done, and in a specific order. As an example, Sprout Tower is now a requirement, as are the Burned Tower, Bell Tower (HeartGold), and Whirl Islands (SoulSilver). The structure when you reached Ecruteak City was also originally more open. While you technically needed to beat Morty before Chuck (surf to Cianwood City), Jasmine (surf to Cianwood City and back), and Pryce (surf on the Lake of Rage), the order for those three was somewhat interchangeable, along with the Radio Tower takeover. I can't remember for sure, but I think the order of events is more set in HGSS.
The game also has more "cutscenes" (aka required dialogue events), and the dialogue itself is a lot wordier. HGSS have a ton of content and love so they're not bad games, but you can definitely see Game Freak's way of thinking and designing games has changed and is becoming more like what we know now. But what we know now doesn't have the content to back it up.
I would love to play a hack of HGSS that made it closer to the original.