(1) There are a few concepts I picked up in yeshiva which are valuable and even though I have fallen away from them, they are still important to explain to those that might benefit.
One is the idea of Bitul Torah. [Not learning Torah when you can be learning.]] In other words you might be a working guy. That is great. But if you do not actually need the money then you are in fact not allowed to work because that is time that could be spent learning Torah.

The does not mean one is required to be learning Torah all the time. Rather it means when time is available then one is in fact required to be learning Torah.

You see this in all the obvious places in the Yerushalmi and the Rambam but somehow or other the idea rarely gets across to people--including me.
(The Rambam considers Physics to be in the category of learning the Oral Law as you can see in his discussion of dividing the day into three parts.))

(2) הכרת הטוב gratitude was another concept I was introduced to right away. The most clear expression of this is the Musar book חובות לבבות Obligations of the Hearts. This you also see in Rashi when aaron struck the Nile River and it became blood.  Rashi says there that Moses refused to strike the river because it had once done  a favor for him.

(3) דיני ממונות [laws about money] was obviously learned well at the Mir, but this also translated into deed as well as learning.

(4) Speaking the truth-the absolute truth at all times with utter total disregard for one's own advantage was another concept I picked up. I could tell lots of personal stories how that concept helped me in miraculous ways during the years.