Ok, let me get back on topic here - 4e Essentials. The reason I think there might be a transition barrier is that the Starter Set is only 96 pages total. The combat chapter alone in the PHB is 32 pages. The full Rules Compendium is slated to be 320 pages. The numbers suggest to me that the rules in the Starter Set might be significantly truncated.
Certainly they'll be truncated. Not many 1st level characters need to know about Petrification. Or Stunning.
Prone? Yes, they'll probably need to know about that.
However, we're talking only about the Starter Set here, not the rest of the Essentials line. Starter sets always leave things out. But once we get to the new Rules Compendium? It's going to include the rules for everything. We know from the previews that Paragon Paths, Epic Destinies and suchlike are in the Heroes books. Rituals might be excluded, but why? To force people to buy the core books *in addition* to the Essentials line so they can run D&D?
Sorry, but no. That's not going to happen.
The Essential line - once past the Starter Set - will be the full game. It'll be the full game explained in a manner better for new players rather than experienced D&Ders.
The only part of it that won't be the full game is that it won't have every class, feat and power in the core books. They are, frankly, not needed. Instead, a new range of feats and powers (as well as a few core feats/powers) will be presented. Which makes these books interesting to old players in addition to new players: more options.
Ultimately, there won't be much difference between:
A player with Player's Handbook + D&D Insider and a player with D&D Compendium + Heroes of the Fallen Lands + D&D Insider
Both players should be able to create any character in D&D, and have the rules to play it.
Cheers!