As a new DM, and having had three different DMs myself recently, I think the biggest thing is: know your players.
That doesn't mean you have to know they're favourite movie, etc. But you should be trying to read their body language and listen to their comments to see how they feel about the game. And it never hurts to ask.
Yesterday I was in a game where one character had a homunculus that died due to an arbitrary DM fiat. Now, it was hilarious in the context of the game -- a roll of exactly an 83 caused it to be hit by a carp that had been shot by an arrow that had missed the original target. But the player himself was a little put out, as that was 3000 GP gone, and his whole schtick was that he built stuff like the homunculus, his golem, etc.
Thankfully, we had some treasure and I, as a player, found him the fungus golem that actually cost less than his homunculus, so all was well in the long run. But if we weren't all friends with well over 10 years of history behind us, it'd be the sort of thing to watch out for.