It is not, at all. Neither d1 nor d2 were as complex as people want to make believe. Why do you believe this would be the case, assuming you in fact believe it?
Skill trees, runes, runeswords, charms, jewels, elemental damages, the way resists worked, the way stats affect class, the number and purpose of gems, damage scaling, weapon selection, ad naseum. Meaningful choices means depth.
There is very little decision-making in D3. In fact, there's more thought process required for the AH than there is in the game. Sure, you could play D2 mindlessly if you wanted to since the PvE was tantamount to D3 MP0. But D2 had depth if you chose to explore it. You have no such option in D3.
Skill trees weren't deep at all. It was basically pick a few prereqs and maximize your core skill, period. Later on this feeling got mitigated with the addition of synergies, but it was still the same thing, in essence. Besides, people would just google the current cookie cutter builds and stick to them. That's why bone necros, hammerdins and trapsins became popular after 1.10 patch, for instance. Charms? More of the same stats at the cost of your inventory space. Weapon choices isn't that different to what it is now: Picking the best weapon to suit your needs, which directly relates to damage scaling. Instead of having a max dps weapon, you would have a hoto with insane caster properties, or a botd with massive melee damage. Meaningful choices mean depth indeed, when one doesn't just make up their existence to pretend they are there.
Jewels, runes and runewords are just more socketing options. Great stuff too. At most you can claim they made whites relevant, but deep? Hardly. You mention gems, what gems were actually relevant? Maybe a topaz in your armor/helm for added MF? The rest were barely used, if at all on equipment. They were just used to reroll stuff in the horadric cube, if i remember correctly.
The only thing you mentioned that actually added depth was elemental damage, which is something i would indeed welcome for D3, since it would directly diversify the rather linear gear progression present in D3.
Since you mentioned some sort of exploration for D3, how many non-cookie cutter builds have you tried to play with? I mean, forget about efficient farming and just focus on how the skills work and how they can be fun to spice up and break tye typocal efficient farming routine. There's a crapload of crazy builds you can come up with in D3, yet people still hold onto this "no build diversity" bandwagon like it was actually insightful, when all they mean to say is waah waaaah not all builds are efficeint baaaa waah waaaah!
. Ever tried to check those random events most people tend to overlook in their obsession for efficient farming? Perhaps you've already listened to the textbooks and lore snippets spread across the world too? How about those usually unvisited random dungeons you can find around the world? All these things tend to be overlooked, because people are often too obsessed with efficiency. You choose not to explore it, deal with the perceived "lack of depth".
Could it use more? Most certainly, but a lot of people will always see the glass half empty, regardless of how many thing sthey add, precisely because the game didn't become the D2 clone they wanted it to be, and can't deal with the franchise simply going in a new direction they don't personally approve of.