What is something that you feel most 2nd lieutenants lack?

Experience... Just like most new Soldiers/Service members!

There are a handful of commissioned officers who never should have made it through the process. They’re easy to spot. Otherwise, all joking aside, most of what young LT’s lack can and is fixed by time and mentorship. The only thing I can’t fix is a bad attitude.

Experience. If you listen to your NCOs you survive and thrive until you get it.

While I was in Germany(my first duty station) the butter bars lacked experience. They all looked toward the senior leadership for advice, knowledge, and assistance. They were great.
While I was in El Paso(my second duty station) the butter bars lacked experience and respect for the senior leadership. They would constantly bicker with the NCOs and it was quite embarrassing to see the NCOs bickering back.
If you go commissioned, make sure you listen to the senior leadership. Just because you technically outrank an NCO does not mean you do not have something to learn from them.

Experience, wisdom, and maturity. But that's kind of expected.

Feedback from PVTs....

Cadet CPL Lamin Conteh Experience. And willingness to listen to their NCOs who have been at it for awhile. I am most grateful for the NCOs that pulled me aside early on and said something akin to you can either do it the way ( your commissioning source) told you or we can teach you. I went with option B.

Thismay not be a popular response but it comes from my own experience both as an NCO and as an officer.
It is almost axiomatic that 2LT's lack experience but that can be fixed by a decent NCO. Something harder to fix is a lack of perspective. Many new officers get caught up chasing minutiae and miss the bigger picture. What is required here is massive quantities of patience and a willingness to let the LT fall on his face once or twice on small stuff -- big stuff has a wider impact and his soldiers don't deserve the fallout that comes from a raging eff-up.
As a soldier, and later as an NCO, you will need to establish a good sense of perspective for yourself and that will allow you to more effectively house-train your LT.

Nobody's all knowing right out of the box, whether in the military or civilian life. Experience, mentoring from senior officers, guidance and patience from NCO's. On the LT's part, don't be too proud to ask.

The ability to realize that a book can’t teach real world experience!!! All the lieutenants I ever met had a hard time learning that what they were taught in a college environment had no real application to active duty life!!! The academic side helped in job position but some of the history of warfare sunk in to deep and they had a hard time adapting to new strategies and combat standards!! Sometimes their ego was their biggest problem!! Instead of realizing that they were equivalent to a buck private in the officer world and needed to learn from the experience of NCO’s and field grades to become leaders!! To become a good leader you first must learn to follow!!