Although I really appreciate webOS' multitasking & Synergy, I doubt those will be the reasons the TouchPad succeeds. These have been strengths of webOS since day one, but weren't enough for webOS to grab significant market share.
I'd say the 3 main reason that will help the TouchPad succeed are:
1) Applications & Devloper support
2) Marketing & Ad campaign
3) HP's vast distribution outlets
-Many people try to downplay the importance of apps, but it really is critical. The UI & many features of webOS are great, but without apps & devs supporting the platform, the TouchPd will be limited in it's functionality.
-The Marketing & Ad campaign is critical. The masses really don't know about webOS, and a great ad campaign is needed to give people a positive perception of webOS & increase consumer mindshare/awareness.
For example, previosuly whenever I showed off my Pre, unless the person was a techie, they usally asked: "What phone is that?". Yesterday I was showing off my Pre 2, and one of my friends said "Hey, that looks like the phone that Manny Pacquiao uses in that commercial". What a difference a good ad campaign makes. The new ads barely kicked off, and already I can say it is having a better effect than palm's previous campaign.
-HP's scale in reaching both consumer & enterprise customers is huge. In addition to the 600,000+ retail outlets they will be selling TouchPads in, HP has a sales force of over 20,000 people that specifically target corporate clients. With those numbers, they will definitely be abe to move tons of TouchPads in both consumer & enterprise markets. This is probably also the reason why they ship 2 PC's a second. I'm pretty sure they have more outlets to sell the TouchPad than any other competitor, including Apple.
So I think these 3 points will probably be the biggest factors in whether or not the TouchPad succeeds.