I think the main thing DC need to learn is that the Marvel films have been good in large part because they chose an overall "lead" director for the universe that had a very good track record of pulling together storylines with dozens of characters over multiple years, and then they had everyone working along that path (with I believe a number of the key people being actual comic writers, or directors who were huge fans of the comics and knew the characters).
That you should employ directors and writers that actually have some talent in making a coherent film. Preferably with several films behind them to prove it wasn't a fluke, and at least one in the "genre" (or work in TV if no film).
That reshooting and editing in large bits when you've already shot most of the film rarely makes for a coherent story, and if you're going to do it, give the continuity people some extra time and money to make sure that if for example one of your main characters has cut their hair since the original shoot makeup can afford a decent wig.
This is a minor pet hate of mine in films, when a character is changing look subtly between shots due to reshooting later on when the makeup/costume isn't quite the same or the actor has put on/lost weight and they're trying to disguise it (it's distracting as I'll often notice something has changed but not be able to put my finger on it).
That it's alright to have some humour in a "dark" film (unremitting grim dark with no humour just gets boring, whilst dark with odd bits of humour can seem even darker without it getting boring/feeling like they're going over the top), but there is a fine line between having some humour and having inappropriate humour for the sake of it because another film had a lot of humour and did well.
I enjoyed suicide squad, but it could have been much better. Visually it was very good, I liked the character designs and costumes (even if some of the characters were weak), but the story was more suited to a short, or a series.
I think even some of the characters were done very well, what we saw of them, but the whole didn't quite add up.
I'll have to admit I thought they got Harley Quinn down very well, very close to what I know of her comic book and animated versions.
On a side note, at least it wasn't BvS
and in the interests of full Disclosure, I've tended to be a Marvel comic fan in the past, most of the main DC hero characters used to strike me as pretty boring with the exception of Batman.