Here are some experiences I made playing Dungeon World, "a modern game with old school feeling".
I was a GM in three sessions so far. I have never played DW before but have had an intro session of Apocalypse World, DW's "father" game, in a Google+ Hangout.
In two games (with different players) I used a Dungeon Starter by Marshall Miller: The Goblin Hole and The Shallow Sea. All of my players have never played DW or AW before but all of them are veteran role-playing gamers. In my last session I went with the default "Don't prepare anything but ask questions"-style.
This is easy!
The dice mechanics are a blast and rolling dice is fast. The mechanical bits of chargen are quite easy especially if you've ever played a game like DnD before. It can take some time to forge the bonds and to establish background and setting. The layout of the Playbooks is confusing sometimes. For example, the Bard has a starting move called Arcane Art where several options are listed, but the prerequisite is stated underneath. It's easy to overlook.
Nonetheless, the idea of Playbooks is very appealing for me. Every character fills his own niche. I think the creators did a very good job with the Fighter especially. In most games I know, fighters are pretty boring but in DW the Fighter has a distinctive feel.
For chargen just help the players go through the checklist in the your book, explain how the moves work so that they may choose the right attributes and point out some of the tags (precise is a good example here, it can be quite interesting for the Thief or the Elven Fighter).
Be on the lookout for...
The questions from the Dungeon Starters didn't help us to establish the world. For me and my players they were too limiting. The questions mostly didn't inspire my players to come up with good ideas. Thus, I created the world more or less on my own, relying on the information on the Dungeon Starters. Because I couldn't get valuable player input the adventures were a bit dull and the charm of creating the world together was missing.
One of my players didn't like it when I turned questions back to him, he said that brought him out of character. This may have been a fault on my part as I didn't elaborate enough at the beginning of the session. However, in Dungeon World there is a certain player empowerment which some players may not be too fond of.
Fights can be quite chaotic because there is no classic initiative. As a GM you need to make sure to give every player a chance to shine. Players who like tactical gameplay with crunchy rules were confused about the free-form combat. I suggest that you explain that all stems from the fiction and if they want to have some tactical advantage they can get it by describing their actions.
Also, monsters can be squishy because they generally have less HP than player characters. If possible, make monsters act first and endanger the players, otherwise it can be too easy to defeat the monsters (unless you want that as a GM). I highly recommend reading the fan-made Dungeon World Guide.
Make sure to make some hard moves!
When players roll a 7-9 and you are forced to come up with a move consider turning back to the player. But be careful not to cross the line.
For Dungeon World to really shine you'll need active players who like player empowerment (and creating the world together) as well as sharing the spotlight. Tacticians and Power Gamers can be frustrated by the lack of clear guidelines for gaining advantage on monsters. Using a battlemap and minis loosely can help to clarify the positions of all the combatants and supports a more tactial mindset.
The most important thing I learned was: It is important to ask the right questions at the beginning of the session! ASK, ASK, ASK! Try to establish as much as possible.
Unfortunately, for me, the Dungeon Starters were more of a hindrance than a help. This was especially the case with the Goblin Hole because the questions and the assumption of a dungeon crawl were confusing for me as a GM. The Shallow Sea was much better. Your mileage may vary but I still recommend creating your own questions/taking the questions from the Dungeon World book for starters.
There are also Noofy's Provocative 20 Questions which might help you.
In my last session most of the players really had fun creating the world and incorporating their backstories into it. As a GM, this is very good for you because you get plenty of plot hooks for your first session.