The three questions blend together so I will sort of answer them all at once.
1. I like to take the stereotypical, allowing the players to go on assumptions and then twisting it, sometimes subtly and sometimes more extreme. This is a great technique I use in my fiction and it works fantastic for gaming. Maybe the orc that is tromping through the forest is not a 1HD easy target. Maybe it's a 5HD orc/troll with regenerative powers wearing the Charm of Gruumsh that grants any weapon it uses to paralyze. This is a deadly example, but a good one that plays on players' assumption. Everyone has the monster manual, but GMs have to remember those stats are just a suggestion.
What to watch out for: Making everything unusual so in doing so you've made the unusual the usual. It's okay to have a horde of goblins for the players to slaughter. That's fun too. A GM needs to set up the situation so that the twist will work.
2. I am very good a winging it. I think this is probably one of the most important skills a good GM can have. I think the best adventures are ad-lib and some nice surprises for the GM as well as the players. Of course to be effective the GM needs a solid knowledge of the system. Ad-libbing is cool, but also I think needs to be done in moderation. A foundation and consistency needs to be established. Some people who have the entire world, cultures, history and geography memorized then it's no big thing.
What to watch out for: Like I mentioned the biggest problem with too much improvisation is inconsistency. Too much and it's difficult to keep track of motivations and people and things and places because you made them up on the spot. Keep good notes or at least have a good outline/structure that you use to keep things from going too wonky.
3. I think what makes me a good GM is I engage the players in interesting story-lines. There is often several running through a single game at any one time and some are the main story arc for the characters and some are those side quests or path splitters. I like to weave a lot of layers and build a complex relationship to the world they're interacting with.
What to watch out for: Over complicating things. Players love to develop their story-lines and will often complicate it enough themselves, GMs need to allow them that privilege. If things slow down a bit the GM can always interject some new interesting twist, but if your players are anything like mine they create enough drama to last several sessions in one sitting.