3 reasons:
The System Requirements are prohibitive. I never been really sure what people in Redmond consider the primary duties of an OS, but for me it is managing system resources and making them available. Managing RAM, the CPU, the GPU and the Harddisk. However, the System Requirements would have you believe the primary duty of an OS is to consume system resources? 1GB of RAM totals half the RAM of your average Netbook. The equivalent of a 1GHz Pentium Processor means I doubt the Netbook will do even so much as finish the installation before giving up (1.6Ghz Atom is too slow for Vista which requires less than Win7 already).
But no having a Netbook nor planning to buy one, and least of all installing Win7 on it... I suppose: who cares?
The second reason why I voted Poor is that the Installer is dreadful. It is slow. True the Vista one is slow too, perhaps even worse. But the Vista one isn't something that should be taken as yardstick except for how not to do it.
Also, it is fundamentally crippled: it won't install to external harddisks connected via USB or Firewire. Uhmm? What kind of OS installer is that? It is after all not like the OS cannot run off an USB drive: for laughs try preparing one with a bootable primary partition and copy over your existing (preferably activated, altogether more ironic) Vista or 7 (on the lower level they are nearly identical anyways) installation... Then thinker with your BIOS so it tries booting off the external harddisk: lo and behold: it boots. It runs. It works. (Been there done that.)
The third is another Installer issue: it is plain braindead.
It is fundamentally broken (but this has been the case with Vista too) it will not install to anything but the first SATA harddisk in a given array. No: you don't have to give it IHCx drivers. You didn't have to with Vista either. Of course it may help; it may not. Putting on different trousers has about as much effect because the OS comes with these drivers by default (the chipset is so commonplace as a southbridge; otherwise you'd often wonder why your harddrives weren't detected since the chipset can not be communicated with without drivers).
What does fix that particular issue though is to open your case, disconnect all SATA cables except those required for your CD/DVD drives and the *one* (this is the crucial bit) SATA disk you want to use for installing the OS. Then the installer *will* work as if by magic (not really: because the only SATA disk left is now the first one in the configuration). Then when you are done you plug everything back in, and suddenly... all the hardware just works (upon reboot) ? To me that sounds like a braindead installer refusing to install to anything but the first harddisk.
Since I can't be bothered to turn of the PC and do cable magic, I shall now further test it from a Virtual Box drive. But my initial experience thus lead me to label it a Poor as in Abysmal Already.