"missed the tutorial". Did you think it wasn't necessary and dove right into a sandbox world, or did you not see the button on the main menu?
Minecraft only allowing you to place blocks next to blocks is just how the UI works. Terraria only allows you to build next to existing blocks, or on background blocks. This is probably because its way to easy to leave a bunch of floating specs around without this restriction. Since Roody:2d has gravity for (almost) every block, and its much more building focused, there is no such limitation.