I can think of one compelling reason. The 3ghz northwood was the end of the line for that platform where as s939 and even s754 had a number of faster chips coming out for them.
But yeah, I don't think northwood was terrible overall. I'd just moved away from Intel during that era and it would have taken a much better product to move me back, not just competitive. You're absolutely right that prior to the introduction of nforce AMD chipsets were a huge weak point. I think VIA was the biggest supplier for quite awhile, I remember rocking an sdram thunderbird setup and never really loving that motherboard.
I think the worst was probably the cacheless celeron. People dump on prescott and it was terrible but they still made a ton of them. And AMD is still making bulldozers. The celerons without cache were so bad they stained the whole celeron brand right out of the gate. Even after the awesome 300A had been out for a long time and the prior ones were dead and buried I still had people telling me how shitty celerons were based on the initial reviews. Hey, the misconception kept the prices down for those in the know. There were certainly some shitty celerons since but I'm talking about during the 300A's heyday I was hearing this.
True enough, I guess what I was trying to say is that during the release of the first 3000/3200+ chips, a ~3Ghz+ P4 NW was not worth moving from unless you wanted to do a sideways move and be ready for more upgrades. At the time though Intel was still promising that Prescott and future releases would remain competitive. We know how that turned out though, with only a couple of S478 Prescotts that sucked butt hahah. And then S775 looked like garbage until C2D came out, and of course C2D wouldn't work on basically any early S775 mobos anyway, making it what I would term the darkest days ever for Intel.
My first AMD64 build was a S940 Opteron before S754/S939 were released at all with the AMD64 consumer chips. I remember being incredibly impressed with it.
I totally agree about the 300A, a great secret for those in the know. 300A @ 450 was as fast or faster than the fastest CPUs on earth at the time (it was released well before Slot-1 P3s I think as well), so you could spend a tiny fraction of the money compared to even a P2-333 and get as good or better performance depending on app. Same core, smaller but had cache on-die running at full clockspeed. This brings me to another interesting point :
Slot-A Athlons had off-die cache for a long time, I think all the way up to the very first 1Ghz models. What was funny about that is that 1Ghz Athlon only had 333Mhz external L2 Cache, which really put a fairly notable damper on things in some regards. Thunderbird, which included both Slot-A and Socket A variants, had the on-die cache and was the super buttkicker everyone remembers when thinking of the Athlon branding. Slot-A Athlon (Pluto and Orion) basically tied up pretty evenly with Pentium 3 models, whereas Slot-A Athlon Thunderbird and Socket-A Thunderbird opened up and maintained a notable lead over P3 and even extended that lead over Willy P4s in most things (I think Quake and encoding were about the only things P4 had the lead in : eg P4 1.5Ghz > pre-XP Athlons on those sole benches).
Oh how I hated *most* socket-A chipsets though. VIA and partners had a bunch of doozies. Weird BSD problems, incompatibilities with certain hardware (SB Live caused a bunch of issues IIRC on some setups), and iffy performance, hindering the awesome Athlon XP. Irongate 750 or whatever wasn't much better, and worse in some regards. Then Nvidia came out, and their 2nd try, Nforce2 .. it was just awesome. The same AXP chip, taken off a KT266 or KT333 board, would really fly on an NF2 board. The quality difference was amazing. About the only impressive VIA chipsets I remembered were the KT133A, which delivered amazing SDRAM performance, and the KT400 at the very end. Most of the interim releases seemed half-assed, particularly when compared to the seamless and rock-stable experience of running a good DDR 845 or 865-based P4 setup. Of course, P4 had it's own crappy VIA and SIS chipset options, along with the horribly overpriced RDRAM 850 and horribly slow SDRAM original 845.