The problems might not necessarily show up in your game, because the places where 4e and 4e.E fail to mesh aren't so much systemic as specific, and depend upon combinations of campaign, characters, builds, feats, magic items and synergies between characters.
So bully for you if both classic and Essentials characters are living in your campaign in perfect harmony--I play in a campaign where both types of characters are used, too, with little difficulty--but the fact that some ENWorld DM's have chosen to run all Essentials or all 4e campaigns, as well as the opinions of several posters in this thread, should be all you need to understand that your experience isn't necessarily universal without someone having to take to time to write a report about it. My feeling is that anyone combining the two is going to have to make at least some small adjustments to make everything work together, (magic items, for instance), and one player's small adjustment is going to be another's pain in the ass.
Also, as someone may have mentioned upthread, the problems wouldn't necessarily appear in a campaign, because DMs can merely hand-wave any issues away. In fact, the incompatibilities and inconsistencies between 4e and 4e.E are more often revealed prior to that, in the character creation faze. Errata is slowing getting everything in line, but if you want to see where 4e and 4e.E aren't working well together, just watch the errata.