To proclaim understanding of such a young tablet market is a fool's errand, but I think there are some additional considerations to be made beyond Gassée's article where he concludes that the iPad is a tease and it cannot fulfill the duties it promised to fulfill.
To evaluate the tablet, it is worthwhile to consider smartphones. Smartphones replaced their predecessors entirely, and with ease. It wasn't a "good enough" solution and the way we use phones didn't have to change. Sure we got all sorts of new ways to use phones, but the same old ability to make calls and send SMS usage was fundamentally still there. All of the things people needed to do, they could do.
The same cannot be said for tablets right now when you measure them as PC replacements. Tablets enable us to do a lot of work, a lot of the same work, and a lot of new work, but it doesn't allow us to do all of the work we can do on a PC. If there is even one critical task you must accomplish that isn't possible or realistic on a tablet, you suddenly need a computer. One tiny little thing can throw a wrench in the gears that easily. Sure a new piece of software with a unique way of interacting with or producing data to fulfill the same need is possible, but it might not be adopted at your company or it might have shortcomings.
So why this massive difference in how things turned out? For starters, the phone industry was an infant when smartphones replaced them. Okay not technically an infant but they never really added usage-changing features, for all intents and purposes people just needed their phones to make calls and send messages. The computer industry is a lot older and has some extremely entrenched workflows. It is much easier to dethrone something so young (dumb phones), especially if you (smartphones) are vastly superior in every single way. Computers aren't young, they aren't dumb, and they are absolutely critical in just about everyone's job, if not their life.
The other thing that complicates this discussion is that tablets are being framed as needing to replace traditional PCs to be successful. I think that is a misguided notion. There is certainly a lot of overlap between the two, but surely it isn't realistic to expect tablets to eliminate the PC industry. It isn't clear where this will all end up. Even though iPad sales growth is declining with no obvious explanation, the tablet market is nicely establishing itself in peoples' lives and won't be going away any time soon.
(My thanks to Jordan Hendry for a thought provoking discussion that lead to this train of thought.)
UPDATE: Updated the title to reflect the tone of the article more, I'll leave the URL to keep links live.