abstracted_methods.js

function getUUID() {
    nano.request(
        {
            method : 'GET',
            path : '_uuids?count=1'
        },
        function (err, body, headers) {
            if (err) {
                console.error(err);
            } else {
                console.log(body.uuids[0]);
            }
        }
    );
}

function createUser(uuid) {
    var usersDB = nano.db.use('_users');
    usersDB.insert(
        {
            'name' : uuid,
            'password' : 'pwdUser1',
            'roles' : [],
            'type' : 'user'
        },
        'org.couchdb.user:'+ uuid,
        function(err, body, header) {
          if (err) {
            console.error('[_users.insert] ', err);
          } else {
              console.log('you have inserted the uuid.')
              console.log(body);
          }
        }
    );
}

does_not_work.js

async.waterfall(
    [
        getUUID (callback),
        createUser(uuid, callback)
    ],
    function (err, result) {
        console.log(result);
    }
);

waterfall_friendly_methods.js

function getUUID(callback) {
    nano.request(
        {
            method : 'GET',
            path : '_uuids?count=1'
        },
        function (err, body, headers) {
            if (err) {
                console.error(err);
            } else {
                console.log(body.uuids[0]);
                callback(null, body.uuids[0]);
            }
        }
    );
}

function createUser(uuid, callback) {
    var usersDB = nano.db.use('_users');
    usersDB.insert(
        {
            'name' : uuid,
            'password' : 'pwdUser1',
            'roles' : [],
            'type' : 'user'
        },
        'org.couchdb.user:'+ uuid,
        function(err, body, header) {
          if (err) {
            console.error('[_users.insert] ', err);
          } else {
              console.log('you have inserted the uuid.')
              console.log(body);
              callback(null, 'done');
          }
        }
    );
}

works.js

async.waterfall(
    [
        function (callback) {
            getUUID (callback);
        },
        function (uuid, callback) {
            createUser(uuid, callback);
        }
    ],
    function (err, result) {
        console.log(result);
    }
);

Or you could just pass the functions as variables..

// Works
async.waterfall([
getUUID,
createUser
],
function (err, result) {
console.log(result);
}
);

Error in your first thought solution is that the functions get called in place since you pass params to them.

// Does not work
async.waterfall(
[
getUUID (callback),
createUser(uuid, callback)
],
function (err, result) {
console.log(result);
}
);