1) an adventure module is published. On the back cover, they spend 44 lines of text telling you they use rule a but not b and module c for combat is required and the magic system the bad guys use is module a again, ad nauseum and they have no room to tell you what the cool adventure is. Inside, the adventure writers have to spend 16 extra pages of notes on "if you don't use this feature, replace it with that feature" stuff.
A reasonable concern, but I am definitely optimistic.
There will presumably two types of modularity: PC/NPC/monster design, and running the game.
Regarding the first, I don't think adventures will make much difference. The players are still going to be able to use their PC whether they created them as basic or as complicated as possible. NPC and monsters provided by the adventure will be pre-designed, and the DM at worst may have to check how some specific high-complexity ability works. However I think most NPC/monsters provided by adventures will be fairly low-complexity exactly so that they can work in everybody's game.
2) I'm a player looking for a new game, or a DM recruiting. Right now, I say "I run 3.5e, E6 version". Anyone who doesn't understand can ask "What's E6?"
Under DnDN I might spend 5 minutes laying out what I do and don't allow or use in my campaign, just to explain WHAT GAME we're playing.
If they can minimize these issues, I'm definitely in favor of where 5e seems to be going.
I don't think there'll be a hundred modules in core. It's very possible that "rules modules" will be just between 10-20, so it might take 10 minutes to explain them. The point is that something like using or banning a single feat or spell is
not really modularity as they intend when they talk about 5e. Modularity mostly regards entire sub-set of the rules, although they can be small sub-sets, but still things such as using a variant for running combat with more details on tactical movement, or a variant for HP and life/death rules with different conditions, or a variant on healing (rests, surges) to allow seamless combat during a day...
Smaller changes (yes/no to individual character options) can be just called House Rules as usual, and a DM who likes using tens of them will still take the same amount of time as in previous editions to explain them all out.