Owen Chadwick, in his book The Reformation, gives a few lists of the vices Erasmus saw in the church:
Erasmus was not fired by a reforming passion or zeal. But his sensible and scholarly nose was otherwise offended by the stink of corruption. He despised ignorance, superstition, obscurantism, and wished to cure them. Because his pen was able to portray those vices in the most entertaining light, he could communicate his own contempt to countless other minds. The diffused effect of writings like The Praise of Folly (1511) or The Colloquies (1518) cannot be calculated.
Educated men were mumbling all these things about the clergy, about monks and popes, corruption and graft, popular superstition and idolatrous practices. Erasmus expressed, and brilliantly, what they were barely articulating; and educated Europe laughed. ... More than any other single man, he lowered the European reputation of popes and clergy, monks and friars, and (above all) of the theologians.
Above all the theologians. He once described a contemporary as 'a scab of a fellow, theology incarnate'. He condemned them as pedants, logic-choppers, manipulators of meaningless notions, constructors of syllogisms, warriors over terms. 'A man might sooner find his way out of a labyrinth than the intellectual mazes of the Realists, Nominalists, Thomists, Albertists, Occamists, Scotists.'
(Chadwick, pp. 32-33)
Erasmus made efforts to simplify theology:
In 1503 he published the Enchiridion militis Christiani (handbook for the Christian warrior), an attempt to expound the lines of this true theology. It was a simpler theology, more primitive, more Biblical, less tangled in logical subtlety and more direct to the human soul, stripped of the layers of glosses and authorities and commentaries.
(Chadwick, p. 38)
He is probably best known for his Bible scholarship. He remains hugely influential in the field of textual criticism, and it was important to him that people be able to read the Bible in their own vernacular. Unsurprisingly then, his hero was Jerome, the great fifth-century Bible translator. Luther and Tyndale based their Bible translations on Erasmus' text. Given his interest in making the Bible accessible to commoners, it should come as no surprise that he also wished for hymnody to be accessible:
Modern church music is so constructed that the congregation cannot hear one distinct word. The choristers themselves do not understand what they are singing, yet according to priests and monks it constitutes the whole of religion.
(Erasmus, Commentary on 1 Corinthians 14:19)
He disliked when religious extravagance and superstition become substitutes for simple faith:
Erasmus and his fellows were impatient, contemptuous, angry with the superstitions of the people. Those superstitions, cults of statues, visits to Madonnas that rolled their eyes or to bleeding Hosts, seemed to be not mere harmless vehicles of a rude devotion, not merely vulgar and credulous, but the bane of true religion. The people cultivated a religion of external acts and substituted pilgrimage, an indulgence, a relic, for a genuine change in heart and life. It is the better side of Erasmus, the concern for true religion, which turned his satire into the severest form of condemnation. 'Perhaps thou believest that all thy sins are washed away with a little paper, a sealed parchment, with the gift of a little money or some wax images, with a little pilgrimage. Thou art utterly deceived.' 'Without ceremonies perhaps thou shalt not be a Christian; but they make thee not a Christian.'
(Chadwick, pp. 38-39)
We kiss the old shoes and dirty handkerchiefs of the saints and neglect their books, the more valuable relics; we lock up their shirts and clothes in cabinets adorned with jewels, but as to their writings on which they spent so much pains, and which are still extant for our benefit, we abandon them to mouldiness and vermin.
(Erasmus, Preface to Jerome)
He opposed paid indulgences, and in his Right Fruitful Epistle in Laud and Praise of Matrimony he indicated that he believed priests should be allowed to marry. His pamphlet Julius Excluded from Heaven portrays Pope Julius II (1503-1513) as worldly, arrogant, and tyrannical.
It's easy to see how well many of his views accorded with those of the reformers. But of course, he came to oppose them and their teachings.
Like Erasmus, many educated men would have preferred the Church to be ridiculed into good sense and efficiency and purity of life. But a man who is holding property will not be mocked out of it. There were forces more potent at work, both to maintain the existing state of the Church, which would not be altered without violence and illegality, and to ask whether the existing state of the Church was not the symptom of a deep-seated and moral disease. There was a celebrated saying of the sixteenth century: 'Erasmus laid the egg and Luther hatched it.' It is certain, at least, that Erasmus alone would not, and could not, have hatched it.
(Chadwick, p. 39)