Brand name thyroid meds are more consistant in the dose. That is, they really do have the actual anount of active ingredient that it says it has. (ie. 100 mcg-100 mcg)
Sounds obvious, but generics can differ by as much as 25% in active ingredient and the FDA is ok with that! (ie. 100 mcg generic levothyroxine could have between 75-125 mcg)
This really matters with thyroid meds because the TSH needs to be kept in a very narrow range and a small difference in the amount of t4 you get can make a big difference in the TSH and how you feel.
Also it is possible that a filler in the generic didn't sit well with you.
Glad you are beginning to feel better.
Sounds like you're just better suited to be on Synthroid. That's great if it's working already.