But 5e still offers so little in terms of ways to mechanically differentiate a character.
Seriously?
I am all about character customization; as a player I live for the ability to play characters mechanically distinct from any other. And to me 5th Edition represents the pinnacle of balance between a 3E level of character customizability that I desire, and a 1e/0E level of ease and fluidity of play plus DM prerogative in running the game that I also desire. To me, 5th Edition feels like a game with the simplicity and fluidity of Basic, the mood, atmosphere, and attitude of 1st Edition, and the character distinctiveness and customizability options of 3rd Edition.
I truly do not understand the opinion that 5E "offers [...] little in terms of ways to mechanically differentiate a character" considering the presence of Skills, Feats, Multi-Classing rules, and a full Background character creation element that grants Skill, Tool and/or Language proficiency, and even a special feature (and even stating in the rules that you can work with your DM to create your own new Background if there isn't one in the book that fits your character concept), and includes Personality Traits, Ideals, Bonds, and Flaws as character attributes which even grant actual mechanical Advantage when roleplayed well. And even on top of all those elements, every single Class has further built-in customization on a basic level by
requiring a choice from multiple Subclasses, such that any party could contain multiple characters of the same Class who are nevertheless quite distinct from each other simply by way of having different Subclass specializations.
To me, all of these provide all the character customization options I could ever want. What more kinds of customization options would you ask for?
However, if you are referring to the actual available options to choose from that have been made available so far, the limited number of Subclasses and Feats published, I can pretty much agree with that. To cover that problem I have found a number of excellent options from DM's Guild that add a good number of very serviceable new Feats that we use in our games, and we also use the Unearthed Arcana articles with new Feats and Subclasses. Also, to add some additional variety and customization options for players we use a houserule (from the
Feats of Heroism PDF from DM's Guild, the primary 3rd Party rule supplement that we use) giving every PC a bonus Feat at 1st, 10th, and 20th level (compensating monster power by giving them all the
Tough Feat, a simple +2 hit points per Hit Die, or sometimes a Weapon Master Feat when appropriate such as special elite Hobgoblins or Drow.)
I also hold out hope that WotC will continue to publish more rulebooks like Xanathar's Guide to Everything containing more Subclasses and Feats.