If your bike fits you well now, it's going to be weird with drops unless you change the stem. My goal was to put the hoods in about the same position as the grips were on my flat bars (though not as far apart). This required a much shorter stem with a much steeper rise. The steeper rise was necessary to get the hoods at the right height. The shorter stem means you've got less distance to achieve the same height so the angle needs to be steeper. I hope that makes sense.

I ended up using some flared cyclocross bars that were wider than typical drops because I still wanted to be able to take the bike offroad now and then. I was pretty happy with the results. Getting the stem right was one challenge, the other was dealing with the front derailleur.

While rear Shimano derailleurs use the same cable pull regardless of whether they are meant for road bikes or mountain bikes, the same is not true of front derailleurs. So STIs won't work quite right with MTB front derailleurs. You can of course switch to a road derailleur but most of those are bottom pull and a lot of mountain bikes use top tube cable routing. You may run into cable pull issues depending on what brakes you have too.

What ended up working best for me is switching to Campy shifters, the type that feature ratcheted shifting on the front rather than indexed. This meant getting a shift-mate for the rear to compensate for the different pull. It was a fun project and I was happy with the results but there were a few challenges.