The real problem that nobody wants to mention is that it fundamentally lacks
role playing.
I know this is a concept that's made fun of -- "haha you play the
role of Mario so it's an RPG!" -- but the problem is right there in the joke: You
must play Mario. And in DE, you
must play a detective. There is no other way to go about the game. You can be a shitty detective and different types of detective, but you are always(..eh..) a detective.
In Planescape Torment you must always play The Nameless One, yes. But these are very clearly different in their expectations. To quote Avellone himself:
Players should be able to play an RPG the way they want, and they don’t need my moral judgments getting in the way of how they have fun. I also am not a fan of pre-determined attitudes and alignments for players-my hope is that at the end of the game, they’ve answered the question, “What kind of character am I really, and how did that depart from what I thought I would be?” I always considered Torment a sort of role-player’s experiment, where each incarnation of the Nameless One had the potential to be a different personality and a different type of gamer
In DE, you are assuming a character with a very predefined role and are shackled to it. In my NHL example from the first page, it has this issue aswell where you must play as a hockey player. In many cases where you think "This isn't an RPG, but I can't place my finger quite on why", it likely comes back to this. Well, why isn't Borderelands an RPG? Because you must -- always, absolutely required to -- play a Vault Hunter with certain social and story expectations put upon the player that you aren't allowed to deviate from.
Consider Morrowind, where the strongest shackles placed upon you are never even confirmed to exist -- the game never tells you whether or not you're actually the Nerevarine.
This isn't to be confused with given a general purpose or a goal, but that you're given one and specifically required to go about it in a certain way that restricts the player's freedom to play the game he wants in an unreasonable manner within the confines of the game itself.
And, perhaps to some extent, this is an argument against many RPGs -- most typically ARPGs -- being considered cRPGs rather than their own separate but related genre. After all, what do Witcher 3 and Wizardry 7 really have in common? There's an interesting argument here for the first Witcher game because you have amnesia, and the lines definitely blur.