The first vilāsa of Haribhakti Vilāsa explains quite a bit about the qualities of a guru.
It begins with the most essential, core quality… HBV 1.35 quotes the Upaniṣads, “tad-vijñānārthaṁ sa gurum evābhigacchet…” The essential qualifications of the guru is…:
1) to personify the conclusions of the Vedas,
2) to be expert in explaining those conclusions,
3) and to be fully dedicated to pursuit of those conclusions.
HBV 38-40 give more details about important qualities of a true guru, quoting from Mantra-Muktāvalī. The guru…:
1) has a pure lineage (referring to the parampara more than the birth family)
2) is pure (isn’t blamable for serious things)
3) is dutiful (doesn’t abandon their responsibilities)
4) is “in āśrama” (may clarify #3, and/or may mean that they pursue spirituality)
5) gives up anger
6) is very knowledgable
7) knows every śāstra
8) is faithful/convinced (of śāstra’s conclusions)
9) does not hate or envy anyone
10) speaks with endearment/ pleasantly (“is nice to hear”)
11) is pleasant in gesture and body language (“is nice to see”)
12) is clean / honest / innocent
13) dresses well (i.e. makes endeavor to please others by appearance)
14) is youthful (may also mean “young”! May also mean youthful-spirit)
15) loves to help every creature
16) is pensive, introspective, intellectual
17) humble character
18) is content
19) is not selfish
20) is deliberate and careful
21) has other good character traits (forgiveness, etc.)
23) resolute (follows through on intentions and conclusions)
24) is respectful
25) is like a mother or father to students
HBV 41 suggests that a guru must instruct the disciple using a balance of both praise and correction.
HBV 42-44 quote Agastya-Saṁhitā about the guru’s qualifications. The guru…:
1) worships divinity
2) is peaceful
3) has no interest in external objects of pleasure
4) understands spirituality
5) can explain the Veda
6) expertly understands the meaning of the Veda
7) both corrects and encourages as appropriate
8) is dedicated to spirituality
9) understands reality
10) understands the true essence of things, understands the heart
11) understands mysteries and secrets
12) can perform rites and rituals with perfect mantra
13) understands how to accomplish things (how to accomplish “yoga”)
14) is minimalist and spartan in personal habits
15) speaks only the truth
16) maintains a household
HBV 45-46 quote Viṣṇu Smṛti, explaining that even if someone has all these qualifications, they shouldn’t go out looking for disciples or advertise themselves. They must only accept disciples out of compassion for and affection for the students.
HBV 47-55 clarify that for Vaiṣṇava dīkṣa the most important qualification is that the guru recieves a Viṣṇu mantra from a proper lineage, and uses that mantra faithfully to worship Viṣṇu.
In his comment on HBV 54, Śrīla Sanātana Goswāmī quotes the pañcarātra stating that it is advisable to reject dīkṣa from a person who is not a Vaiṣṇava, and take the opportunity to receive dīkṣa from one who is.
HBV 56-58 quote Tattva-Sāgara to illustrate additional circumstance in which it is not advisable to accept a person as guru, or in which is may be advisable to reject the guru. These may be taken as evidence that the person is not truly a Vaiṣṇava. This is when the guru…:
1) desires excessive sex and other greeds for external pleasures
2) espouses mundane philosophies justifying vile behavior devoid of good character
3) pursues behavior not fit for their āśrama – which could be evidenced in physical ways like their becoming too hairless or hairy (a gṛhastha being too renounced or a tyāgi becoming too opulent), and/or in their developing bad teeth, discolored lips and bad breath.
4) becomes full of bad traits (lying, stealing, anger, hatred, etc.)
5) who accepts donations even though already well-off
HBV 59-72 clarify that a guru can also refuse to accept, or can reject a disciple under circumstances where the disciple displays refusal to learn, etc. In fact, if a guru does not reject such people, it is evidence that the guru wants the prestige of having many disciples, and is a sign that the guru is fallen.
HBV 73 – 78 clarify that a guru must not accept a disciple before the two of them actually live together for one to three years and mutually decide on their compatibility.
As I see it, if a person has all the above qualifications, they are a perfectly qualified guru and it would be extremely easy to be a very good disciple of such a person. Therefore we should try to find a guru with all the qualities, or at least a great many of them. The core property (as evidenced by the Upaniṣad quote at the outset) is a very clear understanding of the Veda, and ability to clearly explain that. Without this core quality, even with the other qualities, a person is not qualified to be guru.
Thus, the main quality of a guru is the ability to explain the Vedas and clear up doubts various people will have about their conclusions and meanings.
Vraja Kishor das