The problem was that his effectiveness was really only implied; that backstory that you were talking about wasn't really actually stated, and all that he did that was actually shown was shapeshifting into a cloud and advancing on the tower. Compared to the other villains, he barely did anything; NMM, even not counting what she did in the intro to the episode, was actively trying to stop the Mane 6, and if nothing else had a better character design. Discord transformed Ponyville into... whatever that was, in the time it took the Mane 6 to get from Canterlot back to Ponyville, and ripped them apart friendship-wise. Chrysalis invaded freaking Canterlot, took out Celestia, and nearly won. And at the very least, all three of them had decent lines; heck, even Trixie and the Diamond Dogs were better antagonists because of that.