Prova così:
codice:
SELECT value, SUM(value_count) AS total_value_count
FROM (
SELECT conformita AS value, COUNT(*) AS value_count
	FROM voti
	GROUP BY conformita
UNION ALL
SELECT disponibilita AS value, COUNT(*) AS value_count
	FROM voti
	GROUP BY disponibilita
UNION ALL
SELECT gentilezza AS value, COUNT(*) AS value_count
	FROM voti
	GROUP BY gentilezza
UNION ALL
SELECT precisione AS value, COUNT(*) AS value_count
	FROM voti
	GROUP BY precisione
) AS single_value_count
GROUP BY value