Am I the only one who hates the recent switch from top-mounted to bottom-mounted PSUs? Now, instead of the PSU pulling air from inside the case and blowing it out the back, it pulls air from below the case and blows it out the back. In theory that might seem a better idea because the PSU is sucking in cool air, but most PCs are placed on carpet, which means you’re also sucking in dust, carpet fibres and all kinds of other crap that will soon cover the insides of the PSU. Not a good plan.
You can fit them upside down, as I did in my home server, but then if anything falls down (e.g. you accidentally drop a screw while fitting another component, or a tiny sliver of metal comes off while screwing it in) it goes straight in the PSU. If you can’t get it out, it will short out next time you start up.
Another argument used to justify the change is that modern computers are more power-hungry, so the PSU needs better cooling. That may make sense if you’re using a dual-CPU system with four GPUs, In this case, my new i7 system uses less power than the Pentium-4 it’s replacing.