Not what they do, just that the syntax isn’t consistent (do I need quotes around this string or not?), the results are a distraction to get into a usable table, and you need to write absurd loops to perform object-at-a-time data gathering. I’m not talking about running DBCC CHECKDB (necessarily), or turning on Trace Flags, or any cache-clearing commands — you know, things that perform actions — I mean things that spit tabular results at you.
I completely agree. One of the nicest things about SQL is that I can use the same syntax to read metadata that I can data. DBCC commands are a jarring difference.