Agreed.
This was something that bugged me about the "Legends & Lore" article where Mearls first talked about modularity in detail - he made it clear that adding modules would also up the power level of characters. Sure, he also went on to say that the DM should also up the challenge (by using a greater XP budget for encounters), but it doesn't help when the players want to use different modules from one another.
I think the key thing is customisation. In 4e 'Classic', all the characters have lots of choice in their powers; in 4e Essentials some of the classes had much less customisation. However, the two were equivalent in power.
I think that's a good model. The Starter Set (and perhaps even Core Rulebook) should give a fixed progression of fairly simple powers for each class. However, it should also note that these are powers, and that in other expansions there will be the option to swap these for other powers. The supplements can then give lots of options (as per 4e 'Classic').
That way, you get your younger player playing with simple characters, while your more advanced players get a much wider set of options, some of which are inherently more complex. However, both can play together, because the 'Essentials' progression is valid as a 'Classic' build, and is roughly equivalent in power.
(There are, of course, limits to this. If you give lots of options, this will always result in a slightly higher overall power level than the limited simple build. This is inevitable - at best, they can be equally powerful, but that requires god-like design abilities. However, you should be able to get things "close enough" to work... and use errata/revisions/ban-hammer to fix any big problems in the expanded options (not the Core).)
