Traditional commentators make much of these rabbis and the nature and the deeper meaning of these words of praise. One topic though tends to be overlooked. Why does the mishnah say of Rabban Yochanan that "he used to enumerate their praises" rather than the more frequently-found, if prosaic, formula "he used to say"? Surely it is obvious both from their content and their context that what he says about his star pupils is praise--and mishnaic style suggests that the Tannaim prefer not to state the obvious.
A possible explanation is that the mishnah is teaching that each of these five attributes is something that can be a criticism if found in a person of lesser quality. Thus:
"A sealed cistern" is a positive quality in a talmid who retains only his rabbi's teachings (See Sukkah 28a), but a person who retains everything, regardless of its quality or substance, is like the sponge that absorbs everything indiscriminately (Avot 5:18).
A rabbi "makes his mother happy" and this is a good thing when her happiness is contingent on the performance of good deeds, Torah study and the like--but not when it is the consequence of the mother selfishly or jealously refusing to let go of her child (cf Bereshit 2:24: "So a man shall leave his father and his mother and cleave to his wife, and they shall be one flesh").
"Pious" is great, but not when the chasid is a chasid shoteh (a "pious fool," the man who responds to the sight of a woman drowning by saying that, as a religious person, it is inappropriate for him to look at her, even though that is the only way to save her: Sotah 21b).
"Sin-fearing" is also great, but a person can be paralysed into inaction by his terror at committing any sin if he goes into business, takes on a role of communal responsibility or even commits himself to marriage.
To be an "ever-flowing spring," full of ideas and enthusiasm, can be a precious asset. However, such a person can also be extremely annoying. If you have ever sat in class or in a shiur with someone who is always interrupting others in order to get his answer in first or to second-guess the flow of a line of argument, a person who cannot control his effervescence and simply will not shut up, you will know how destructive this asset can be.
In other words, Rabban Yochanan is emphasising that, when he uses these terms, they are words of genuine praise.
