When it first started I saw the players go for their builds, it was a TvP on metal if I recall correctly. I was confused by some of the things in the early game, mainly why the castors were stressing his stim timing that much. I didn't know what it did. However, as soon as it was done, and the castors said "Stimming forward" when the marines had lightning bolts over there head and moved quickly, I figured it out. Combat shield was a little harder, but I figured it increased defense/hp or something.
Although I didn't know everything about each unit in the game, they mostly made sense to me.
The midgame was the same thing with blink. "The stalkers blinked right on top of his army!" was all I had to hear to know what it did. +1 Attack and +1 Defense were also clear to me what they did. Medivacs would clearly heal the Terran army, colossus would do splash, vikings would counter them.
I got the basic concepts of the game: Expand for more income, kill works to weaken opponents econ. While there was some things I didn't understand in that game (IE, Mules and why castors were stressing their importance.) I got the general gist of the game and enjoyed it.
I watched the rest of the B/O3, and then it was the next game. TvZ.
The early game was pretty similar to the TvP game, with Terran getting stim and combat shield. Zerg was new to me, but with ling speed being called "ling speed" it was obvious what it did. Banelings were weird at first, but obvious when I saw them be used.
The things I didn't understand were creep and inject. I saw no reason to have creep, and had no idea what it did. Inject also didn't make sense to me, and why people would do it, or why the castors would talk about missing injects.
The midgame were Mutas vs Marine Tank Thor. Mutas made sense, I just didn't know they did splash. The game ended shortly after that.
After that, I loved this game. Although I never played Starcraft 2 before; I knew about 50% of the game from watching it, and the other 50% didn't matter to me.
That is why I love the game that is Starcraft 2.