Draghi didn't realize was how in-love markets have fallen with QE.
What you do at a central bank is only half the equation. The rest is much you can convince markets and the public about its effectiveness.
Markets were wary of early rounds of QE in the US and Japan, unsure exactly what it would mean and how to trade it. That forced the Fed other others to reach for ever-larger numbers that would grab headlines and convince markets. As the Fed actually made the purchases it began to sink in just how much it would drive yields down and hurt the currency but markets didn't believe it until they saw it so that lengthened the time it took to be effective.
The second era of QE is here. It took awhile but now traders are firmly in tune with the QE playbook. You could see it in the most-recent BOJ moves as they quickly crushed the yen and sent Japanese stocks to decade-long highs.
With the ECB the market no longer treats QE as an 'experiment' but trough of trading profits to gorge on. It's been a race to frontrun every cent of Draghi's money; pushing German 10-year yields below 0.10% today, just 5 weeks into the program. "Dump the currency and buy the stocks" has turned into a no-brainer gold rush when similar QE moves would have been met by unbelievers five years ago.
If anything, markets are in danger of overestimating the impact of QE at this point. When crafting the program and choosing the €1 trillion target, Draghi looked at the size-of-QE-to-GDP ratios and rounded up in order to make a splash. What he underestimated was how in-love markets have fallen with QE. He could have announced a program half the size and had the same effect.
All Draghi is doing now is forcing the ECB into a corner where it has to buy negative-yielding bonds and fuel a bubble more than it needs to. But if you're a trader, there is still plenty of room at the trough.