I made a list of all the optional rules and all the random tables in the DMG. There are at least 64 optional rules. Some are rather minor; some have multiple parts. There are 174 random tables.
I was not around during the playtest. Could those who are disappointed in the lack of modularity point to actual words said by the developers on when this modularity was promised and what that modularity would entail? I have not read the 3e or 4e DMG but compared to 1e, 2e, B/X, and BECMI 5e is the most free form of the lot and those DMGs were no slouches in empowering the DM to do whatever.
I don't believe it was promised either, but I do believe it was stated as a design goal, and that set some expectations. I'm too lazy to look for a link on their redesigned site (if that's even where I got that from) so if you don't believe me on that, it's ok. Suffice to say, I don't think we were lied to, and perhaps I assumed too much. Whether it's their fault or not, I was a bit disappointed though.
While optional and variant rules do make the game more modular, it's nothing different than we've seen before, and so I expected more. And I certainly don't believe tables add any sort of modularity (though I do love them).
My impression was that the rules were going to feature many dials and levers so that you could set up a campaign with explicit options like a specific level of magic, and that you could specify explicitly a level of detail for rules like combat. It's not like you can't do these things with 5e, you absolutely can, but the modularity design that I was personally imagining was supposed to make it more explicit and easier to communicate. For instance, instead of listing all the optional and variant rules that you use, you could simply use keywords like "Low Magic", "Slow Healing", and "Highly Tactical" to specify what type of game you were running, dials and levers. The options would be presented in packages so that you could mix and match, perhaps not every little rule, but at least groups of rules. I was hoping for something like dials and levers rather that you had to set, rather than what we have, which is a base game with some options. Another concept was that the Basic rules could be thought of as the only set of mandatory rules, and all the rules in the PHB and DMG would be optional. Even though that wasn't specified in the PHB, it could have been laid out in the DMG.
I didn't have a firm idea of what it was going to be like, because they never said, but I did think it was going to be more configurable in a structured way than past editions, and that doesn't seem to be the case, with the very notable exception that there is a Basic edition that you can default to. (which of course has still been done before with BECMI, sort of...5e gives you a more complete Basic ruleset, which is awesome).
Regardless though, I'm happy with how it turned out.