Here are some pearls of wisdom for aspiring SC2 commentators, coming from a vastly experienced guy who casted two games with Special Endrey once.
WC3 players: If one of the players you are commenting on happens to be an ex-WC3 player, don't forget to mention that he has excellent mincro. People on the internet are stupid and have very shorty memories, so they will likely have forgotten the last time you mentioned that a WC3 player has good micro. Point it out even if they don't have particularly great micro, because how could they possibly be good at the game without having great micro, right?
All players are excellent: Never forget to mention that every player you commentate on is very good. Just because you're commenting on the best league in the strongest esports country in the world doesn't mean that calling the players "very good" is redundant. Even if you're discussing the first round of an open tournament, there's 50/50 chance that Meg@Sn!per.X is "very good," right?
Leery/Weary/Wary: Despite these words meaning completely different things, to a commentator they are 100% interchangable, especially if you are Day9.
Vocabulary: Are you the type of person who thinks long words are hard to say? Do you not really know very many words to begin with? Good news: esports commentary is definitely for you! Long words take up too much time - precious time which could be spent yelling "ohhhhhhh!" People can already see what's going on on-screen, why would they want you to talk about the game?