Most blocks are used as part of some fairly standard process, and should come with standard settings. (E.g. block after third vandalism warning: block for 24 hours, allow talk page editing, allow registration; block for unacceptable username: block for infinite time, allow talk page editing, do not block IP, link to username policy in block summary.) Currently administrators have to add these settings by hand; often they are not aware or not bother and so the block ends up being confusing or too weak or too strong. If they do bother, it makes blocking more time consuming.
It would be nice to be able to define block templates for standard cases ("template" being used in the colloquial sense here, not template pages), so the administrator would only have to select the block type and most other fields would be autofilled.
Ideally this would be a core feature, but for the time being, it's something that a gadget could easily provide, too.