Of course, Debian is not for everyone. Nothing is for everyone, even though some would have you believe otherwise. Here is a deliberately short list of possible disadvantages (kept short because the author believes in the power of manipulation as much as the next guy).
This is quite a valid point, especially in the case of a Stable system. Packages can be even a major version older than in other distros. Why is this a problem? You will not get the latest and greatest functionality. Why might this be good? Because you can be sure that such software is unbreakable, or as close to being it, as possible.
That said, it is possible to get newer packages in Debian, by switching to the testing or even unstable repositories (these are meant for the more experienced user, who does not mind if things occasionally break). You will of course never get as fresh stuff as from a “bleeding edge” distro, like Arch, but you can stay on a safe middle ground, between stability and new features.
This is the disadvantage of not having PPAs… (Only true if you are not willing to compile from source, but quite honestly, who is?)
Yes, newer kernels come out for newer hardware. If you have hardware so fresh that it’s still warm out of the factory, you might run into problems. Unless you want to install a newer kernel, which you can, quite easily, even in Debian.
A major disadvantage can be the lack of a central "Control Panel", and the fact that a lot of configuration needs to be done via manually editing often obscure config files. For most systems, this should not be an issue, as Debian tends to work out of the box, but for some more exotic setups, and some brands of laptops manual configuration might be necessary (such as WiFi, Bluetooth, and other tech that should really be invisible in the 21st century).
Of course, this guide is here to help you in most situations, but not all of them can or will be covered. Fortunately, the internet is full of resources for those times something goes astray.