• Is there any real-world difference between "je ne l’aime pas" and "je n’aime pas ça"?

Yes. L’ is a definite, singular pronoun (which replaces either le or la in this sentence) while ça is an indefinite, abstract pronoun that doesn’t need to agree in gender or number with what it stands for.

• Can they be used interchangeably?

Generally, no.

For example, you can’t use ça when referring to someone:

Mon voisin, je ne l’aime pas.

Mon voisin, je n’aime pas ça.

You can’t use le when talking about more than one thing/person:

Les grèves, je n’aime pas ça.

Les grèves, je ne l’aime pas. (should be: je ne les aime pas. but it is less idiomatic that the form with ça and would mean you dislike specific strikes already mentioned.)

• Also, as a bonus, how would slang/informal speech modify these? I’ve seen things like "je l’aime pas" and "j’aime pas ça" … are they valid?

They are definitely valid and not slang. Skipping ne is standard in spoken French.

I Sometimes there is no difference.

1. If you are talking about a person you can never say “ça”. You can however use “ce” in the idiomatic turn “c’est”.

   • C’est un être avare, je ne l’aime pas.

   Note that you could also answer with “C’est un être avare, je n’aime pas ça.”, but then the interlocutor would be given to understand that what you don’t like, more specifically, is greed (“l’avarice”) and you can use “ça” as you are implying a state of affairs (see next).

2. When talking about a given state of affairs you must use “ça” if you haven’t given it a name and if it is merely explained or described, if you don’t refer to it as a named phenomenon.

   • Il y a trop d’inflation, on ne sait jamais si les prix élevés sont dus à des augmentations générales ou s’ils sont seulement la conséquence de pratiques abusives, il faut alors se renseigner avant d’acheter, je n’aime pas ça. (“Le” wouldn’t do here.)

   • — Il faut construire un quadrillage de fil rattaché aux bords de la déchirure et le renforcer en utilisant assez de fil pour reconstituer une sorte de tissu et ça prend longtemps, je n’aime pas ça. (“Le” wouldn’t do here.)
     — Ah ! Le travail de ravaudage ?
     — Oui ! Je n’aime pas ça/Je ne l’aime pas.

3. When talking about specific actions that have no name you use “ça”.

   • As-tu déjà mangé des insectes grillés ? Non, je sais que c’est devenu très acceptable de nos jours, mais je n’aime pas ça. (“Le” wouldn’t do here.)

   • — Avez-vous un intérêt pour la pêche à la truite ?
     — Non, je ne l’aime pas/Non, je n’aime pas ça.

   It is well to be aware that there is not here a general pattern of usage; for instance, if we use the verb “faire” instead of “aimer” it is not true any more that “le” can’t be used (that “le” is not idiomatic , in other words).

   • — Roulez-vous en bicyclette en vous asseyant sur le porte bagage au lieu de sur la selle ?
     — Non, je ne fais pas ça./Non, je ne le fais pas.

4.When talking about a specific thing you do have a choice.

• — Est-ce que tu aimes la/les sauterelle/s grillée/s ?
  — Non, je ne l’aime pas/Non, je n’aime pas ça.

Here is an extract showing the possible use of « les » for several item (although, all denote a particular art).

From André Malraux, l’homme des univers: actes du colloque, Paris-Grand Palais, 5, 6 et 7 décembre 1986

Of course, « ça » could be used (Bien entendu, il n’aime pas ça.), and there is no real difference; I couldn’t even say that « les » is more formal.

Note   The 3 last categories could be reduced to a simpler scheme of two: things that are not named but only explained or described take “ça”; when a name is used both “ça” and “le” (“la”, “les”) are possible choices.

II The forms considered are not at all reckoned with as slang, they are just informal. They can be substitued without problem, except that, of course, of not being formal enough in certain circumstances; they are not acceptable in formal discourse of the written type. Nowadays, they have attained to some degree of acceptability in some of the spoken discourse that is not too formal.

I’m adding an answer because I disagree with the accepted one and I have a different take than jlliagre.

To me they’re not interchangeable, and the difference is not in person vs things (you can say "je l’aime pas" for a movie among other things), but unique, specific things vs general things.

Je l’aime pas is for "I don’t like this thing in particular":

Ce film, ce candidat, son progamme électoral, cette voiture, ton pote avare, ton gâteau, je les aime pas.

J’aime pas ça is for "I don’t like this type of thing":

Les films de SF, les candidats d’extrême droite, les voitures anciennes, les gens avares, les gâteaux aux fruits, j’aime pas ça.

It can get tricky when you’re using the plural, and there’s just enough things that you could be talking about all of them, but also about them as a kind of things. For example :

Les films de Tim Burton, je les aime pas.

Les films de Tim Burton, j’aime pas ça.

Both work and have roughly the same meaning. In the first sentence you’re talking about all the movies as a group of specific things, in the second one you’re talking about Burton movies as a type of movies.

It can get trickier:

J’adore Batman, mais les films de Burton j’aime pas ça.

Here you’re saying that you don’t like any Tim Burton movies, and consequently you don’t like the Batman ones, even though you like Batman.

J’adore Batman, mais les films de Burton je les aime pas.

Here, it’s implied that you only dislike the Batman movies from Tim Burton, not necessarily all of his filmography (the sentence "… les films Batman de Burton, je les aime pas" is implied).

Note that in some sentences with plural, both forms don’t necessarily work:

Les amis de Vincent, je les aime pas.

here you’re talking about a set of specific people, it would be weird to use "j’aime pas ça".