A big thing is like, don't overstep yourself. Don't try to be someone you aren't. If you're not an ex-invite superstar, don't start giving overly critical analysis like you are one, because your analysis will be wrong, and you'll look like an idiot to anyone who knows jack about the game.
Don't try and be funny. Funny shit will happen in tf2. It will speak for itself. If you have a good co-caster, little funny moments will happen between the two of you naturally. It's not something you should think about or try to force.
In that same note, don't try and hype a situation up. I always roll my eyes when casters just start screaming or in some cases even pretending to lose their shit about crap that isn't even a big deal. Some people eat that shit up. But those people probably watch the casters you aren't so fond of and are just fine with them - because let's face it, most tf2 players don't actually know very much about tf2 and are easily satisfied. From your post, I'm guessing you don't want to be some run of the mill caster loved by the OMG SIGN MY SCATTERGUN part of the community, and thought of as an absolute joke by people who actually play/understand the game.
So, for real, don't try to generate hype that isn't there. It's really fucking cheesy. TF2 is a game that is full of clutch and insane moments. You don't need to hype shit up. Crazy shit will happen on it's own. You just need to catch it, and cast it.
Keep shit real. Don't be fake.
People don't expect you to be perfect, just be you. You don't need to be all knowing - but when you don't know something, you need to know where and how to find it. Don't feel like you have to go it alone. Cast with someone who compliments you, and cast in a way that compliments the person you cast with.