the reason why i believe led ppl to think that vista is a failure is coz of huge expectations.
consider this: windows 95 was a huge leap from dos + windows 3.11. however, since it was the first of its kind there were lotsa bugs (yes, i remember reporting a few to MS myself). when win98 came, it was integrated wid ie4.0, had small but significant interface changes and had most of the bugs ironed out. win98se was a major upgrade on the driver front wid support for WDM. it was more stable than win98 or win95 and even to this day win98se can be seen on many many old computerz.
however, this day when we had a mature OS in the form of winXP SP2 and applications like media player 10, ie6 for winxp and software like talisman, window blinds people had a huge set of resources at their disposal. so for a layman, whats new in windows vista? the aero, only? for him/her the underlying indiscernible new things (like the bsd borrowed network stack, new driver model etc.) mattered the least. the average joe expected much more from vista and wid things like the 'delete bug' (tho it didn't affect all), DRM rumour/confusion etc. added more fuel to fire. this is what led to vista's comdemnation and its comparison between to win98's success, in my opinion.