I have been vigorously pro-generics since the beginning, and this is the reason why: I want generic data structures. Arrays&slices and maps are great, and they really are the 90/10 solution a lot of the time, which is precisely why putting direct support into your syntax for the two of them is so very, very popular, but that other 10 comes up.

Plus, there are some generic data structures that will really work well in Go, like, for instance, an immutable tree. Granted, it'll still take some care to use properly in Go as it does not have "const" or anything like it, but it can still be done. The problem I have is not with accidental mutation, but that I just don't want to sit there and implement the immutable tree code. (Trees are great, but they're really tedious to write in the best of times, and nightmares to debug in the worst.)

I'm not terribly interested in trying to jam functional programming into Go; I may make light use of map/filter/reduce but even if this was fully implemented it would still be a fairly unpleasant experience (function that return "a value and an error" aren't much fun to map and can't hardly "chain" at all). But I've missed being able to just grab a particular data structure a few times.