You can delete every other line in the current buffer using the following command.
There is a fairly elegant way in vim to delete every other line in the current buffer. Why would you want to do that? I don't know. Nevertheless, here it is:
:g/^/+d
This will essentially delete all even numbered lines. If you'd like to
delete all odd numbered lines, delete the first line in the file (ggdd)
and then run the same command as above.
This syntax is a bit awkward, so you may be better off going straight for a
macro (e.g. qqjddq5@q or qqddjq5@q).