"Hold the line!" That's the new rallying cry for the core Windows development team. Add new features. Tweak existing ones. But whatever you do, don't make Windows 7 any slower or fatter than Vista.
I have little doubt those are the marching orders for Windows 7, given the tight release timeframe of 18 to 24 months plus various reports of early Milestone builds. More ambitious changes would risk another - and potentially fatal - Longhorn-style delay. Windows 7 will be exactly what the internal Microsoft working title conveys: the seventh (actually fifth) generation of the Windows NT code base - the same code base that forms the basis of Windows XP and Vista today.
Shocking? Only if you're one of the deluded Save XP die-hards who bought into the whole Windows 7 uber alles mystique. For these lost souls, the next Windows is more than just another version. It's a true panacea, a conduit through which they can pour all of their anti-Vista angst. Don't like UAC? Windows 7 will fix that. Frustrated by Vista's sluggish performance? Windows 7 will run circles around it.
Reality check: Windows 7 will be a lot like Vista. In fact, it'll be more like an extensive Service Pack (think Windows XP SP2 and/or the various NT Option Packs of yore) than a major new release. Big ideas and big new features are what got Microsoft into the whole 'Longhorn reset' mess in the first place.
This is actually a good thing. Despite the criticisms leveled against it, Vista isn't really flawed in any fundamental way. Yes, it's slower than XP - but that was to be expected given its more complex code paths. Likewise, the 'girth' issues were somewhat inevitable. Meanwhile, the hardware base is slowly catching up to where it needs to be to support a more complex Windows OS.
I'd even go so far as to say that, if Vista were launched today - with the SP1 tweaks and improved device driver ecosystem in place - it would fare a lot better than it did. But hindsight is 20/20. The future, in the form of Windows 7, is all about shipping an incremental follow-on to Vista that shores up the NT code base once and for all.
The good news is that this also makes speculating about the next version's runtime behaviour a lot easier. After all, if Windows 7 is just Windows Vista with some performance and usability tweaks, it means we can deduce a lot about the product's system requirements and compatibility with the installed base by examining performance and usage data collected from systems running its immediate predecessors, Windows '5' (also known as XP) and '6' (also known as Vista).