Well, if any Jehovah's Witness said something bad about a convention, or the Watchtower, or the Governing Body, s/he would be labelled as an apostate. It's as simple as that. Surely, they can say, "It was okay." However, if anybody hears him say that, what will the elders think? What will the congregation think? They will all think that this person is spiritually weak. Accordingly, what are Jehovah's Witnesses encouraged not to do? They are encouraged not to associate with people who are spiritually weak. Worse yet, if anybody is seen as an apostate, s/he will have to meet with the elders who will arrange a Judicial Committee. A possible decision that could be made as a result of such meeting is disfellowshipping. Any Jehovah's Witness who is disfellowshipped must be shunned by other active Jehovah's Witnesses.
No Jehovah's Witness would recognize this; however, I believe it is always fear-induced speech whenever a person says how good a meeting was. (Did you notice that Jehovah's Witnesses say the exact same thing about meetings?) Of course, it is not a conscious fear; rather, it happens on an unconscious level. It is continuously repeated to Jehovah's Witnesses that if they reject the organisation, and such would include saying anything bad about the organisation, they are in effect rejecting God himself who allegedly appointed said organisation. As a result, they are in fear—unconsciously, I would argue nonetheless—that they can lose their opportunity of living forever in the Paradise Earth.
Of course, it does not always happen like that. The thoughts I'm sharing with you are rather simplistic and do not necessarily reflect each though pattern within the Jehovah's Witness members. Human minds are unquestionably complex, and I would argue that, at times, such unconscious fear plays a lesser role within the Jehovah's Witness pattern of thinking.