1. Cpu, bw, ram, disk, ddos response, support tickets with dumb questions, etc. are all finite resources that the customer pays for. Despite this there are usually not codified limits on most of these except for ram, bw, and disk (and bw is way overpromised for most users). Instead, hosts rely on behavior of normal clients doing normal things statistically averaging out to result in costs that the host can set prices by. Some people use all their ram or disk (rarely both), others underutilize, it evens out. Trying to codify too much invites obnoxious behavior. Better to tell people to just be good neighbors.
2. Ram and bw use tend to be low or bursty for most users and steady (high) for some. E.g. my ram usage is usually low but sometimes I run a compilation or something that uses a lot temporarily. Someone running a database probably uses their whole ram allocation most of the time. This all averages out.
3. Cpu also tends to be low/bursty or steady/high, but steady/high cpu users are basically not allowed on vps's. I run multi-day analysis tasks at 100% cpu which is why I got into dedicated servers.
4. Disk is not really like the above. There are few bursty users of disk space. Clients tend to use a steady and maybe gradually increasing amount, that may or may not be much of their allocation. But it won't fluctuate much. So here, hosts can observe total usage and add capacity as needed. I had thought LEB wisdom of last year was "don't oversell disk" but I guess it is a normal practice. I have a $15/year Ramnode vps with 50gb disk, that's probably oversold. I have it almost empty at the moment.
5. Having underprovisioned ram/cpu/bw on a host node results in crappy vps performance which is the usual complaint against overselling. Having underprovisioned disk has no effect unless the host actually runs out of space, which results in vps's not being able to save files, creating disruption. I think this it's relatively rare for any host to ever let this happen. They monitor total usage and buy more drives if they have to. So "reserving" space isn't protecting from shortages, it's just increasing host costs and therefore (indirectly) prices. Thanks a lot bro.
6. Hosts generally offer multiple sizes of plans, so if you didn't buy the smallest one, that probably means you probably DO use more of some resource than the smallest one offers. It's unusual though to use more of EVERY resource than the smallest one offers. If I buy a 500GB storage kvm, chances are I'm going to really use most/all of the space, or else I'd have bought a 250GB or a normal VPS. But there's less likelihood that I'll use the 512mb ram that comes with it. If I buy a 128MB $15/y vps, there's good chance I'm running a vpn/proxy/small server and don't need a lot of disk, but maybe I do. So it's nice if they say "50gb allocation" and it's there if I need/want it, like the 500gb bw allocation (I might use 10gb tops). Again it's fine for hosts to figure out the average case and provision for it. Storage server disks will probably be more fully utilized than normal vps's, etc. I have a 1gb ram, 150gb disk vps at ipxcore and I'm using most of the disk but very little cpu and ram, because it's mainly a storage server. If ipxcore observes this is typical for users of these plans, it's fine if they provision their box accordingly.
7. The oversold underperforming vps's from certain notorious hosts are just crap to begin with, I don't see why anyone buys them. None of this discussion really applies to them much.
8. I do like the idea of lots of people storing cryptographic random data on VPS's, and transferring it over the internet between servers, as a way to confuse PRISM. But I think it's a bit unsociable to actually fill VPS allocations of substantial size this way.