Keyword arguments and attr_accessors are my favorites. Very simple but very powerful.
Being able to make my own version of attr_accessor is awesome.
Memoized attributes are handy.
Treat objects as value objects.
fail if this, return if that style is awesome sometimes.
The way boolean operators work so that this_or_that = this || that is possible is awesome.
.select(&:this?).map(&:that).reduce(:+) always feels magical. Try that in PHP.
There are a lot of favorites it seems…
Not very favorite:
this || that or ||= doesn’t work when this = false, work around that.
Shared mutable state lurking everywhere (hey, freeze!).
Eval methods are hard to read, so I prefer define-methods instead.