Lots of opinins but not much metallurgical knowledge.....reminds me of hotroders thinking something is better if its made out of billet instead 6061 AL (same thing). Where's that crazy smilie?
Carbon steel is actually a misnomer, in many industries carbond steel is refered to a mild steel alloy that isn't stailness. What our knives are made of is a medium to high carbon tool steel with a enough carbon that it can be hardened (all steels have some carbon). Stick with asking for carbon steel at the kitchen store or they won't know what you're talkning about, but thats the truth of it
Hardening means heating it past the critical tempurature (roughly red hot) and quenching it (differently mediums for different alloys, ie water, oil, air). After hardening, the steel is 'tempered' which means its reheated to a much low temp to 'let down' the steel or make it less brittle. This also reduces hardness, so the maker wants to create the right balance - different tools get different tempers depending whats expected from them - ie an impact tool is tempered at a higher temp to let it down more so it doesn't shatter. Left dead hard after a quench, the blade would be too brittle - it could shatter if dropped sort of thing.
Incidentally, this is what the japanese laminated cutting tools are all about - leave the inner tool steek very hard and use soft outer ducticle steel to give it strength.
Carbon steel knives were not created for sushi. They predate any stainless knife which is essentially a perfermance compromise - not has hard a steel (which IS edge holding ability) but the don't rust.
Carbon steel can be made extremely hard. even slightly harder than HSS (high speed steel) whose advantage is hardness at heat (up to red hot). This is an important foot note as grinding does expose the steel to very high temps - not all over but where the steel molecule meets the abrasive. Temps are high enough to effect the temper. This is why HSS is preferable for say a drill bit or even a chisel so it can ground and why you have to carefull with carbone steel if you're grinding so you don't wreck the temper
Anyway, maybe more than you wanted to know, but thats facts around the differences.