Remember what really happened there - the original Longhorn builds pre-5xxx were based off from Windows XP.
Then 5xxx and after were based off from Windows Server 2003, a far more stable OS than XP (I know, I know - it's a server OS, but still).
So, a reboot was necessary though.
Longhorn had great ideas but the performance/stability was horrible. They had to scrap it and build something with a more solid base. Unfortunately, with time constraints and a reboot after years of development, they weren't able to put together a solid platform by the time it went gold.
The end result was a decent OS that was missing 2/3 of the new features that were promised. Then, OEMs put it on underpowered machines with bloatware and didn't bother updating their drivers, thereby ruining the experience for everyone. When you look at Windows 7, it finally incorporates most of the things that were supposed to be in Vista.