In Srimad-Bhagavatam, Srila Kapiladeva, while instructing His mother, has given the following characteristics of pure devotional service: "My dear mother, those who are My pure devotees, and who have no desire for material benefit or philosophical speculation, have their minds so much engaged in My service that they are never interested in asking Me for anything--except to be engaged in that service. They do not even beg to live in My abode with Me."
Rupa Gosvami states that there are six characteristics of pure devotional service, which are as follows:
(1) Pure devotional service brings immediate relief from all kinds of material distress: The Lord gives His word in Bhagavad-gita that He will protect surrendered souls from the reactions of all sinful activities.
(2) Pure devotional service is the beginning of all auspiciousness: A person who is engaged in devotional service in full Krsna consciousness is to be understood to be doing the best service to the whole world and to be pleasing everyone in the world.
(3) Pure devotional service automatically puts one in transcendental pleasure: In the tantra-sastra Lord Siva speaks to his wife, Sati, in this way: "My dear wife, a person who has surrendered himself at the lotus feet of Govinda and who has thus developed pure Krsna consciousness can be very easily awarded all the perfections desired by the impersonalists; and beyond this, he can enjoy the happiness achieved by the pure devotees." Happiness derived from pure devotional service is the highest, because it is eternal.
(4) Pure devotional service is rarely achieved: Krsna can easily offer a person material happiness or even liberation, but He does not agree very easily to award a person engagement in His devotional service. Devotional service can in fact be attained only through the mercy of a pure devotee. In the Caitanyacaritamrta (Madhya 19.151) it is said, "By the mercy of the spiritual master who is a pure devotee and by the mercy of Krsna one can achieve the platform of devotional service. There is no other way."
(5) Those in pure devotional service deride even the conception of liberation.
(6) Pure devotional service is the only means to attract Krsna: Krsna is all-attractive, but pure devotional service attracts even Him. This means that pure devotional service is even transcendentally stronger than Krsna
Himself, because it is Krsna's internal potency. The symbol of devotional service in the highest degree is Radharani. Krsna is called Madanamohana, which means that He is so attractive that He can defeat the attraction
of thousands of Cupids. But Radharani is still more attractive, for She can even attract Krsna. Therefore devotees call Her Madana-mohana-mohini--the attractor of the attractor of Cupid. To perform devotional service means to follow in the footsteps of Radharani, and devotees in Vrndavana put themselves under the care of Radharani in order to achieve perfection in their devotional service.