In my opinion a great developer is the one who finds a solution to a problem. It doesn't matter which technology he/she uses. If your organization is already using some tech then you expect the developer to use the same tech so you don't have to hire 10 people for 10 different techs. Any any decent developer should be able to look at any code and figure out what it does and be able to tweak a few things here and there.
More important things are proper comments, readability of the code and documentation. If there's a bug and you are not around, someone else should be able to fix it by looking at your notes. The bug fixer doesn't need to be a guru of that tech.