Well, aside from being able to select infinite amount of units- (that would remove the necessity of mastering extremely hard 1 to 0 hotkey skills.)
You can also hotkey groups of buildings- which removes the necessity of mastering the skills of quickly producing what you need by going back and forth while using the first mentioned skill.
By using tab, you can also switch between unit types within your select- which means you can keep several types of units in one group- no problem. Although, ideally- I've found it it's difficult to control if you have 3 or more different caster types.
These are the 3 major ones. Though a bit small, and minor- and you would already know- if you hotkey SCV production to mineral lines, they automatically mine- unlike that of SC1.
This ultimately reduces a ton of time in game from going back and forth from your base. In essence, many of SC1's skills are actually going to harm your game play efficiency. For example, if you had a habit of hotkeying medics and marines seperately, which you should've in SC1, then it will become harder to control medivacs and marines in SC2. They're meant to be grouped to one hotkey in SC2.
You will notice, all the major skills required for high level macro has just become very easy. I mean, you literally don't have to use more than 3 hotkeys for units, and just 2 for buildings. That's 5 in all. And that's nowhere near hard to use. And even in middle of battle, you could just bring up your buildings and press a few ms and as and have it keep producing no problem. And you wouldn't even have to look back. You don't even have to pull back.
In SC1, when I used to play as Terran, I had to group from 1 to 8 and 9 0 for scanners. I had to use location keys for macro production. In comparison, SC2 is x9999999999 easier. In a TvZ game, I had to pull and push marines and vessels, and make sure medics didn't run into lurkers. I had to constantly press F3 and F4 in order to keep my flow of marines, and constantly regroup. I had to practice for weeks in order to make no mistakes on scanner hotkeys and vessel hotkeys- which would be 8 9 and 0. Well, you get what I mean.
None.