Um, no. You've got it completely backwards. They had all kinds of lofty ideas like WinFS that were scrapped because of time constraints. They've just stripped away feature after feature. So Vista is mostly XP with a new graphical interface and some other minor improvements. Hardly the revolutionary new design MS proposed originally.
'Complete rewrite' my ass. It's still based on old Windows NT code, just like XP is. You don't rewrite an enormously bloated and complex OS like XP in such a short period of time as this. That's why Vista is just an incremental upgrade.
Actually if you want to be pedantic, it was originally called "Longhorn" and its development basically started afresh, building on the Windows Server 2003 codebase, taking only the bits that is intended for the actual OS release. So they didn't take XP and update it calling it Vista.
It's file system is no longer NTFS but now WinFS, which is more like a data storage system based on relational databases, so vista has a much better idea what's on your volumes.
So why did they keep the XP Start button and general look? Well they did intend to completely redesign the GUI, but it was decided to keep a basic similarity to XP so not to completely confuse and scare off their present user base, Crazy innit, but not all people are as computer literate as us you know. I know one person who places her icons on the desktop in specific places, and if something rearranges them sher is completely lost!
The visual similarity is why a lot of people keep saying Vista is just XP+, but it really is a completely different OS. It's a considerable leap in tech.
You may from the above think I am an MS fan girl, but I'm not, one of my servers run Ubuntu! It's just that I'm a tech in our University and as a result MS OS's are a huge part of my job, we get it all first hand. I don't think Vista is the best OS around, and I have a lot of gripes about it, one of them is the activation side of it, for us in the past we had a general blanket licence for all our XP systems, now we have to have a specific licence server which apparently keeps track of all our systems and licences. Not good for our staff who possibly do not connect to work via VPN etc. I'm not looking forward to doing the Vista MCDST though, SIGH.