Synergy, fantastic multi-tasking, great gesture usage, stupid-simple development platform, resolution-agnostic UI being the main ones. Note when I say fantastic multitasking, I really mean it. I half-heartedly wish I had it on my PC. Also, the use of gestures built into the OS was brain-dead obvious and simple, yet until the Blackberry Playbook came around and stole it, it was the only one doing so. It removes all the pointless buttons for actions like going back and forward, and even removes the need for the home button. It's actually faster.
webOS has only had two faults from the very beginning:
1. Performance. I feel if webOS came out a couple years later, it would have rocked everything. But it is currently held down by the hardware too much. It's not unusably slow, it's just not on the same level as an iPad.
2. Apps. Half of this is the fault of performance, and the other is the platform. Since it was designed primarily as an HTML-based platform, there are significant limitations in what could be accomplished. They only just began to rectify that when the Touchpad came out, but it was too late. It did support standard binary apps as well, but those were almost entirely games.
Also I don't agree with Jon`C at all about no one wanting tablets (yes it's made by Apple, but that doesn't automatically make it win. See also: Apple TV). You don't sell 30 million of the things just on a fad. And frankly, it's completely replaced my laptop. It is actually faster than most of the things I'd use my laptop for, and it has a highly reliable battery life. The fact that I can go on the road, watch a movie, and still have 8 hours left is such a freeing feeling. Only a tiny few laptops can come to that, and that's usually at the cost of performance or size.