Starcraft 2: Hotkey to go to previous selection

During the install, the Adjutant describes the events of the first game in an overview fashion to get you caught up. In addition, they do a good job of giving you key information from the first game throughout whenever they reference events from it.

Starcraft II does have its own plot that is not entirely dependent on the first game, so they went to lengths to ensure that you need not have played the first one to fully enjoy the second one.

I would argue that if you're just starting out, grid hotkeys are more efficient since you can memorize either which key it is or where on the unit-card the button is located (visual cues tend to help memorization) - and if you don't know the hotkey, you can find it without mousing over the button.
Also, all the keys are on one side of the keyboard, meaning you won't have to move your hand as much.

That being said, the vast majority of players use the default keys either because
A) They don't know grid exists, or
B) They are already used to the default layout (grid was not an option in the beta), and/or were already used to the very similar layout of Starcraft 1; there really is no benefit to them learning a new layout if they already have the default hotkeys memorized. Most competitive players would fall into this category.