You can't do real work on the iPad version of iWorks.
Again, this is okay with most people due to the different expectations one has with a tablet over a PC. If all you need to do is update one small item on the iPad, it's doable.
Making Excel simpler just for simplicities sake would be a mistake for Microsoft. Exposing the right amount of simplicity for the various tasks is what they should be aiming for.
I certainly wouldn't want to use a version of Excel that was designed for a PC on a tablet. It would be a horrible user experience. The only way to make it workable is to write something from scratch. Sure, it could read and write .xlsx files, but under the hood, it would need to be very different.
they could make iOS able to run OSX apps
I think that Apple has shown that they are actually going the opposite way. More and more of OSX looks like it was ported over from iOS.
One of Microsoft's problems with Tablet PCs in the early 2000s was that they were PCs. Because of this, they needed to have all of the horsepower to run Windows and still be portable enough to use as a tablet. This meant that they were always expensive. iPads on the other hand don't need to run a full copy of OSX, so they can be much lighter, smaller, and cheaper (than a Mac).
The best line in the Gruber piece is this:
You can’t make something conceptually lightweight if it’s carrying 25 years of Windows baggage
This is why Microsoft is going to have issues using the same code base for a tablet and the full Windows 8. It has too much extra stuff. A tablet doesn't need all of that stuff. This is one of those categories where having raw power isn't as important as being lightweight and using what you have as efficiently as possible.
Microsoft may be primarily selling to corporate clients and OEMs, but they will still be competing with iOS and Android tablets in terms of mindshare and expectations.