Main issue from the IGN review seems to be controls, but I don't remember hearing that from a lot people who downloaded the demo.
In the demo, there's only three basic shapes to draw. The Eurogamer review also mentions that drawing shapes can be frustrating.
Morphing is The Wonderful 101's killer move, and it's nearly brilliant. What lets it down is the touchscreen, which is fine with a stylus but - let's be practical here - you can't comfortably play The Wonderful 101, which uses every button the gamepad's got, while holding one. The least inconvenient option ends up being your index finger. The gamepad's almost up to it, but not quite, confusing weapons like the whip and claw and often losing your attempted shape halfway through. As the number of weapons increase and you start switching between them mid-combo, this gets frustrating.