Kylo Ren was definitely the best character in the film and a pretty decent antagonist overall. However, as a huge Star Wars fan (jesus god knows how long I've been roleplaying that stuff, be it tabletop or via forum), I was overall disappointed with the film. The conflict felt like they just wanted to throw a bigger, badder Death Star at us, without even the buildup to make it feel dramatic (also it was basically destroyed in the exact same way). The storytelling around Poe's not-really-death was horrifically lazy, but Finn definitely was the worst character in my opinion. His characterization was abysmal. He went from a Stormtrooper questioning the morality of the First Order to a part of the Resistance cracking jokes as he shot down his former comrades and engaging in a typical Hollywood-level relationship tease with Rey.
Personally, I didn't like that JJ Abrams did nothing with the universe that was already set up for him. Nobody expected him to actually take the EU into account, but the degree to which he flagrantly brushed off a universe that's been established over the course of three decades was nonplussing IMO. I was paying attention to the backgrounds and recognized exactly 2 species from the rest of the series: a Mon Calamari and a Sullustan that somehow managed to sneak in there. He pretty much threw a bunch of aliens as background pieces without taking any actual opportunity to make use of them as characters, with the exception of Maz (prequel garbage class) and Snoke (no opinion thus far). I feel like this is the influence of Abrams' prior experience with sci-fi universes, Firefly and Star Trek, which are both a lot more inclined towards this kind of interaction with the setting. Continuity is king in these series, something that Hollywood doesn't seem to understand in its efforts to milk a little more cash from the cow, and continuity didn't really have a place anywhere in TFA except as a nostalgic nod to the characters from the OT.
Rey and Kylo have the potential to be interesting as they're explored in the rest of the trilogy, but TFA did a shitty job of making me interested in their backstory or their drive. They pretty much went unmentioned, a little less in the case of Kylo but especially for Rey, whose past was pretty much ignored to set her up as a Skywalker and Kylo's cousin. Also, the Republic pretty much existed to get nuked by the third Death Star... supposedly the books that they always publish expand a lot on the in-universe historical backdrop behind the movie, but what use is that if it never makes it into the finished product?
The movie was just too derivative overall. Even the opening crawl echoed A New Hope, and while some interpreted this as an homage to the original trilogy, it didn't really come across that way for me. I mean, Rey is literally Anakin: a (quasi)slave on a desert planet with a knack for piloting. It's obvious they're setting up all the familiar themes, making it so Rey embodies the light side of her family's qualities and Kylo is more channeling Vader and Anakin's dark side at the moment, but... it was all too lazy.
Of course, I'm waxing negative here. Overall I'd give the movie a 5. It was better than I expected going into the theater, because it didn't kill my enthusiasm for Star Wars, but I'm not looking forward to the generation of fans that are introduced to it through this trilogy either.