1. Netbooks are slower than the laptops they're used to
2. Watching HD video on most netbooks isn't all that great (this is improving)
3. Standard desktop user interfaces don't translate well to a small form factor device
4. Battery life sucks (this is also improving)
5. Many netbook owners complain about small keyboards, low-resolution screens and poor pointing device implementations.
Don't get me wrong, I do not like netbooks myself, but a lot of people do and I can see why.
1. Netbooks are definitely fast enough for most tasks they are bought for (web surfing, emails, watching DivX videos, Skype). Yes, Flash is a bit of a laggy affair on them, but you can see how much people "need" Flash if you take into account they are buying iPhones and use them for web surfing.
2. HD is probably very low on the list for most netbook buyers. The screen is too small, you wouldn't see a difference anyway. Also, netbooks don't have built-in BluRay (and people know this when they're buying them) and HD on the web is just starting. No one buys a much more expensive device just to watch YOUTUBE videos in HD. The HD factor is still like 2-3 years down the road.
2a. You can already buy Ion-based netbooks that are well fit to play HD content. They don't play a major role though.The next Atom generation (due in a few weeks) is supposed to support HD acceleration, too. We'll have to see if they keep their promise.
3. True. We've seen attempts in that direction, but they all failed.
4. No it doesn't. Battery life compared to notebooks is awesome, that isn't just a recent development. Getting 7-8h out of a Samsung NC-10 is no big surprise. Don't expect a tablet to magically double or triple that battery life somehow, because the device would get too thick and heavy (i.e. unsexy).
5. Many of the first netbooks had crappy keyboards. Have you typed on the recent Samsung netbooks? Nice keyboards! Resolution will obviously not be substantially higher on a tablet, so I don't know why you bring this up. The one big advantage that remains in this department would be the touch screen.
My guess: CULV subnotebooks will take away substantial market share from netbooks, because they're still small and cheap and allow for more tasks since they're twice as powerful. Tablets? I'm sceptical. Probably too expensive to be a long-run success.