Why Opteron (briefly) succeeded
It was better than Intel's offerings at the time it came out, but AMD only had the advantage over Intel because of two huge Intel mistakes.
One, they pursued the dead end Netburst architecture, believing they could goose it up to 10 GHz by the end of the decade. They probably could have (by exposing the double pumped pipeline this would have been rather easy, in fact) but it turns out almost no one wants PCs with 200 watt CPUs. Their engineers obviously understood the relationship between frequency and power, so clearly they must have thought that the curve of ever increasing power demand for desktop CPUs that had held for 20 years would continue to hold. Perhaps if CPUs weren't "fast enough" for most people it would have continued, but once they were people valued quiet for desktops and battery life for laptops far more than Intel's engineers had figured.
Two, they purposefully held back offering a 64 bit ISA for x86, because they hoped to keep x86 32 bit only and force everyone to their proprietary Itanium CPUs over a decade's time as lower and lower end markets hit the 2GB/4GB limit and needed to go 64 bit. They assumed AMD could never get any support or backing if they developed their own 64 bit x86 ISA, similar to how their attempts at SIMD instructions like 3DNow! went nowhere and AMD was eventually forced to license MMX and SSE. If they hadn't make the concurrent mistake with Netburst, maybe that would have held true, and we'd all be using Itaniums right now.
When Microsoft announced support for AMD64, Intel was caught off guard since they hadn't notified them of this in advance. Itanium never recovered, and Intel was forced to be compatible with AMD's instruction set for once.
Interestingly, Hans De Vries discovered evidence of 64 bit support in Intel x86 CPUs that predated the release of AMD64. Perhaps there was a war in Intel between the x86 people and the Itanium people and the x86 people put it in hoping to get management approval to enable it, but it never was and nothing about it was ever published.