Things like not being able to watch a movie, or even play some video games without missing huge parts of the story, or sitting through a conversation with a group of friends trying to constantly "catch up" to a topic that I missed a few minutes ago, or trying to be self aware enough to not change the subject on people too quickly because my mind has already changed topics twice during their conversation.
I love reading, and it's almost 100% out of the question when I'm not taking my medication. Somewhere around the 5th time of re-reading a paragraph trying to remember where I trailed off only to realize that I just did it again makes it really not a great experience.
Even the worst case scenario of the medication severely increasing my chance of heart attack, stroke, or Parkinson's wouldn't make me stop taking it, because it's given me a life. And I would rather live a shorter happier more fulfilling life than a potentially longer one.
I still remember the first day I took the medication, it was like somebody finally turned off a white noise machine in my brain and I could finally do all the things I've wanted to do.
I texted my wife after my first dose that I heard someone having a conversation near me and that I could hear them, but I didn't know what they were saying and I didn't care.
This was a revelation for me.
I recently chatted with a shrink who said that ADD patients typically _can_ focus on the things they want to do but struggle with the things they don't want to do. I strongly suspect he's mistaken - I don't know a soul who finds it easy to focus on filing their taxes (except sadistic accountant-type friends).
Nothing is more frustrating than having all the "motivation" and desire and curiosity for a task but none of the "ability" to make it happen.