Pros:
Getting to meet new people due to a common interest- This is especially the case for me with fandoms like Harry Potter & KH, which are some of my main ones. Though I love meeting new people in RL I've got anxiety, and this has made it hard for me to make friends in school or at work in the past. As a tween my anxiety was especially bad, so the only way I felt comfortable talking to others was the internet. I've met some of my best friends due to online communities that center around a movie franchise/TV show/etc, so I really owe fandoms for that.
Fanart/Fanfic- Though I'm really picky about both, it's amazing to me that people can be so artistic through fan art and fanfic. I'll admit when I was first introduced to it I thought it was unoriginal and pointless, but I was so wrong. It's wonderful. It enables people to express themselves and since it's based on another work you've already got an entire audience to give you feedback to improve your skills.
Metas/Theories- Oh my god. This. Probably my favorite part about it, honestly. This can't really be done so much with fandoms whose story/show/movie have run their course (but it still happens, just not as often because they're usually metas). Theories are really fun to write and read, and while theres not a lot of fandoms that have got plots that allow the fans to make good guesses of whats to come, the ones that do are a blast. Some people go the whole nine yards and just write a theory from scratch, and then some bounce ideas off each other to make a monster theory. Surprisingly, it doesn't even matter if it's right or not-- the only thing that really matters is if has/had the potential of happening. And of course, the fun that went into it. Metas are really enjoyable as well. Analyzing plots/characters/symbolism to make sense of things is an eye opener to the intentions of why writers do the things they do. Some of this sounds boring-- but between TV show hiatuses, it really kills the time.
Cons:
Ship wars- We all know ships are due to preference, but if you think about it, that preference has a lot to do with what you want or have experienced yourself. So insulting a ship is not only dumb because you're just causing an unnecessary riot, but because shippers often ship what they do based on what they want for themselves. It's really easy for it to get personal. There was this fandom I was in that had them a lot. Ship 1 was about a chick and her old baby daddy. He left her, ruined her life-- came back a decade later, and started trying to make a family with her. Ship 2 was about the same chick with this other guy. The other guy had a really bad past, did some terrible things, but the chick inspired him to be a better person and he ended up admiring her and falling in love with her. There were a million and one wars that went down. The fact was, most people that shipped Ship 1 were people who had a fall out in a relationship and still wanted it to work (or ended up making it work) and people in Ship 2 were people who wanted to inspire others. Whatever. But it's interesting to note the reasons behind it. And it just goes to prove you REALLY DON'T have a right to tell people what to like or to judge them for what they do.
People who have their heads so far up their ass that they refuse to acknowledge that YES, their favorite villain/character really screwed up. And NO, their redemption arc won't be easy. And NO, just because you like them and they had a "terrible childhood" doesn't excuse all the people they wronged/killed/robbed/whatever. YES, they have the potential to be a good guy. NO, they're aren't one yet and they won't for a while.- Self explanatory. It happens too often. And it's especially annoying when they want to shove it up in front of everyone's face.
Those rude people that believe their opinions have been approved by celestial beings and think they're 100% in the right. And likewise refuse to accept other opinions.
People who can't have a good argument, when they asked for one.
People who don't tag their spoilers or don't make it clear that they're a blog/site that doesn't tag them.
And when "fandom" starts to turn into "fandumb".