No there isn't anything wrong with the only speaking black character being a comedic role and being the "heart" of it. Though in the same breath Star Wars wants to beat it's diversity drums, they did bait and switch with Finn. They purposely got the media out that Finn would be equal to Rey in the story and had him with the iconic weapon of Star Wars. Instead we got a pretty typical plucky comedic sidekick. Even worse they had to go and make him the janitor (I cannot tell you how much I hate that joke). Star Wars is great for representing white women but let's not start praising the diversity when other minorities are only represented as the status quo. I'm half Jamaican and half Pakistani, I'm still waiting to see one South Asian character in the series.
How is it a bait and switch with Finn? I don't get that line of thinking. We don't know if Finn is force sensitive, but everything we saw of him in the trailers he did in the movie. It wasn't like Metal Gear Solid 2, where they actually inserted Snake as the playable character into scenes where in the actual game, it was Raiden, to keep the surprise.
Rey embracing her destiny was always the plot of the movie. Finn was always the secondary lead. Disney and Lucasfilm really wanted to keep Rey's big moment a secret because it's a pretty cool moment in the film.
Finn fighting Kylo and losing isn't a knock against his character. It actually highlights one of his major character traits and the point of his arc. He was running the entire movie. He kind of has the Han Solo ANH arc, in that he isn't particularly interested in the Rebellion/Resistance, but it's a means to an end (paying off his debt to Jabba/getting away from the First Order). Finn does a few key things in the latter half of the movie that cement his heroic arc:
1) he turns back from leaving Takodana after the Starkiller Base destroys Hosnian Prime. He could have just hopped on the ship. It was no longer his problem. He could have disappeared, and let the chips fall where they may for the Resistance and the First Order. Instead, he ran to Han to let him know it was the Starkiller, which up until that point, the Resistance didn't seem to know existed.
2) after the battle on Takodana, when Rey is captured by Kylo, Finn makes another brave decision: he informs the Resistance about Starkiller's whereabouts, and volunteers to lead a mission to disable the oscillators shield generator, so the Resistance can disable Starkiller. This is very important for Finn's arc, because this is the moment he decides to fight the First Order. I mean that in he's not fighting them becausentheybare keeping him from escaping. He's fighting them because they have to be stopped. He's no longer running, and once they arrive on Starkiller base, he is committed to the end, no matter what.
Han: Are you sure you're up for this?
Finn: Hell no.
3) when Kylo corners Finn and Rey, and Rey is incapacitated, Finn has another choice to make. He can hand over Rey and Anakin's light saber to Kylo, and maybe Kylo will let him escape on the Falcon with his life (ha! Not likely...), or he can, against a more experienced foe, stop running and face his fears. He bravely and awesomely chooses to fight for Rey and himself. He wasn't going to run anymore, and he wasn't just going to roll over and die. If he was going to fail, he was going to do so looking it in the eye. It was the defining moment of his character arc in this first movie. Him winning against Kylo wasn't nearly as important as him facing Kylo in the first place.
Finn was very much equal to Rey. He's the second primary character we are introduced to (Kylo was first, and Poe is a secondary character). We meet Finn, then we meet Rey. That's treating him on equal footing just by film standards. We pretty much always meet the most important characters first.
While Rey was piloting the Falcon, he manned the turrets. Without him providing cover, Rey would have been shot down. They both celebrate their victory. Finn has more screen time with the returning cast than Rey. You can argue he bonds with Han more than Rey did. They have more of a back and forth than Han and Rey, and they go on the Starkiller base mission together. He has more one on one time with Chewie, even if it's played up for laughs (and a throwback to Chewie choking Lando). Chewie carries Finn to the Falcon, as well as loads him onto the medical platform and follows him after they return from the mission, implying that Chewie feels for Finn in some way. Chewie only goes with Rey to find Luke because the Falcon needs a copilot, and I doubt R2 qualifies.
He got to wield the light saber and used it for more than just cutting open a tauntaun. He used it to fight both a first order trooper and Kylo Ren. He killed one trooper with the light saber, but got hit with the stun baton when fighting the second. He lost to Kylo, but I already explained why him winning that battle wasn't the point of the fight.
Besides, Finn's arc is far from over. He had to get from first order defector to hero in this first movie. The wounds he's sustained in this one will no doubt inform his character in episode 8. Much like Luke in ESB, Finn is probably going to level up. His force sensitivity is up in the air, but even if he's not, he's a fantastic character. There is nothing wrong with the hero character having a sense of humor. People love Spider-Man because he's the plucky Everyman that takes it on the chin. Victory mixed with defeat. Finn didn't lose every battle he was in, but he ultimately comes off as a hero, even if he didn't stand victorious over Kylo Ren (which wasn't either of their arcs in this first film. Kylo lost to Rey, and that's going to inform his character in episode 8 as well; my thoughts on it are that Rey isn't going to be so fortunate in their second encounter).
The sanitation thing I never got the hubbub about. Yes, when he worked on Starkiller base for a turn, he was in sanitation. And? At the start of the movie, he was a member of Captain Phasma's murder squad. He clearly isn't just a "janitor." Military men often have different jobs throughout their military careers. They start low, work their way up, and often they get kitchen duty, latrine duty, guard duty, etc. it's not all badass heroics and warfare.
They played his sanitation assignment for laughs, but again, so what? We've already seen that Finn was a capable soldier (taking out Tie Fighters with both Poe and Rey, killing some First Order troopers with a blaster and light saber on Takodana, being vouched for as not having any problems by Captain Phasma after his defection). I find the harping about a throwaway line of him being in sanitation completely over-reactionary. Most of the typical black men in film tropes in this movie are turned on their ear.
And the diversity and representation in this movie go way beyond just a white woman and a black man. As I mentioned before, this movie has many different people peppered throughout the film. There are female first order soldiers. There are black, Asian, Latino women and men working for the Resistance. Oscar Isaac, who plays Poe, one of the most popular characters in the movie, is Guatamalan.
For Star Wars films, which has been almost exclusively white men and aliens, this type of visual diversity is unprecedented. What I'm trying to say is; baby steps. The Force Awakens has opened the door. I'm black and Samoan. There's no Samoan people in the movie, but that's not the point. I'm just happy to see the casting director of the film not just defaulting to white, whether the role is big or small. It's very possible that in a future film there will be some Jamaican or Pakistani actors and actresses present. EDIT: I forgot the queen of Naboo in Episode 2 is Pakistani. I actually think Padme's right hand man in episode 2 is Samoan.
What TFA showed the industry is that a big blockbuster can star non-traditional leads, and bring in the audiences. Star Wars was always going to make money, but the dialogue happening around this movie is how much people love Finn, Rey, and Poe as characters. That's very cool.
Finn didn't need to be T'Challa to be a cool character that audiences can relate to. Hell, in the climate we live in, where the dehumanization of black men in the media is prevalent nearly every time a police officer kills one, I find it wonderful that the most relatable and human character in the film is a black man. Baby steps.