They were going for an "Over the Edge" sort of thing (from The New Batman Adventures), I guess, but it didn't work as well. What made "Over the Edge" so powerful despite being all just a dream was its strong foundation in character. The revelation that it was Barbara's dream wasn't a copout but actually made it more meaningful, because it made it a manifestation of her fears and doubts about her life choices, revealed something about the character that we hadn't known before, and led her to make an important decision about her relationship with her father.
But here, the only revelation was that M'Gann is a much stronger telepath than we thought. While that technically constitutes learning something about a character, it's not something that carries emotional weight or provides a strong resolution to this particular 22 minutes of storytelling; it's just the setup for future plot developments.
And what character stuff we got was mostly stuff we already knew -- M'Gann loves Connor, Wally's in love with Artemis, there's tension between Kaldur and Robin about leadership. We did see Connor make some progress toward reconciling with the Superman legacy, but that's the only significant consequence to the "dream" on a personal level, and even it feels inconclusive as long as Superman remains offstage.
Otherwise, the episode was well-made and had some effective moments. My favorite was the Iris West-Allen news report. It was cool getting the camera's-eye view of the whole thing, and I loved the subtle interplay between Iris and the Flash/Barry, the way they were trying to keep their relationship secret while on live TV but you could still see the little moments of affection.