When the Rolls-Royce (RYCEY) Trent 900 engine failed on the A380, it spewed out parts in all directions. While some fell harmlessly to the ground, others shot up. Some of the debris punctured the wing spar, which is the main structural element of the wing. This caused the gaping hole that passengers saw in the top of the wing from onboard the airplane.
The crew was bombarded by dozens of error messages following the failure, some more important than others. For example, two of the fuel tanks on the airplane were punctured and began to leak highly-flammable fuel. Worse, fuel couldn't be dumped, nor could it be transferred between tanks as usual. So as it leaked out, the plane grew tail-heavy, making it very difficult to control.
Multiple system failures
But that wasn't all. Hydraulics control the maneuverability of the plane and one of the two systems was cut. This meant that enough of the systems were available to land the airplane -- but just barely. Other cables were cut, and the other engine on the wing next to the failed one couldn't be shut down. It was controllable, but the pilots couldn't turn it off.
Considering everything that happened, it could have been much worse. When that engine exploded, it could have punctured a fuel tank in the wing, not just the spar. Had that happened, it's far more likely that there could have been some sort of explosion to bring down the airplane.
As is always the case in accidents, several things have to go wrong for disaster to result. While several things did go wrong on the A380, these particular events fortunately didn't combine to bring the airplane down. But you can certainly now understand why Qantas is grounding this airplane. Since this is a known issue (thanks, Rolls-Royce) that could cause a repeat performance, there's no sense in taking the risk. When the fix is made, the airplane should be safe, but now it's unclear how long it's going to take.