I'd say that CPUs run ~about as hot today as they did back then; power efficiency wasn't a primary concern. Regular computer users didn't care much about the thermals (it either worked or it didn't), and CPUs didn't do the thermally-limited clock boosts that are common today.

GPUs could be spooky hot. While the cards themselves didn't seem to care at all, the heat emanating from a 3dfx Voodoo3 card (I've had a 2000 and a 3500TV) could be enough to make other nearby cards stinky. Fan brackets and other hacks, from now long-defunct small companies like 3dfxcool, were pretty common.

We had PCI, then AGP, then PCI Express -- with overlaps. Woe be to those who errantly picked the older standard for their shiny new build; they would forever be stuck with their new hardware on the older bus, or with an even-deeper hole in their wallet.

There were other unfriendly things. RAM compatibility was weird; I remember a time around the turn of the century when double-sided RAM became inexpensive enough that was cheaper to buy 256MB of double-sided RAM that the PC may only use half of, than to buy 128MB of single-sided RAM.

Cables weren't always keyed, and when they were keyed it wasn't always in useful ways. Plugging a floppy drive in backwards was a common experience. A 40-pin IDE cable might have a key pin blocked off, while the hard drive had all 40 pins populated, and the two wouldn't fit together without modification.

Burning CDs was often a slow-moving disaster, involving feeding expensive blanks into an expensive drive installed in the machine every half-hour just to hope that things would work this time so you could finally listen to some of those MP3s in your Ford after you spent hours downloading them.

Unless you were foolish enough to buy an external drive, instead: As USB was attrociously-slow, those burners usually plugged into the printer port in what could be most-charitably described as an awful fucking mess.

And even if we weren't burning CDs, we had that pesky analog cable to deal with -- neither end of which was necessarily standardized -- so we could play Total Annihilation and listen to its (rather excellent) soundtrack at the same time.

We still had things like sound cards and modems and IRQs to deal with. NICs were often separate from motherboards. BIOS flashing felt like a dark and dangerous art. The fan headers often had different shapes, and usually only some of them were capable of ramping fan speeds.

At one point around that time, we rather quietly shifted the main power draw for new motherboards from the 5v rail to the 12v rail. Existing power supplies weren't always ready for this and it led some down expensive and unexpected paths.

I enjoyed working with PC hardware back then. But it was all pretty unfriendly, and I haven't even mentioned yet how much fun SCSI was to deal with.

Nowadays: We buy a fish tank and install a motherboard and power supply into it. The smart builder has already got the NVMe drive, CPU, and cooler fastened down into place in their purposeful mounts before this point. The fan headers all work. If we saved enough pennies, we add a GPU -- and it slots into the same x16 PCI Express socket that we've been using for over 20 years now. The NICs (often a plurality of them!) are built-in. There are no dip switches or configuration jumpers.

All that's left is to plug it in and install an OS. Easy.