There are a lot of people out there who like to mind their own business and just go with the flow. I have nothing against that, I'm a person like that myself, but if you play in a band, staying silent and agreeing on anything your bandmates propose might be harmful to your experience with music. There aren't that many people who are actually this shy, but it's still important to express your opinion on decisions affecting your band.
Let's start with a short story about my life. A while back, I was in a band that played traditional Finnish melodic metal, ranging from melodeath to power metal. I founded the band with a guy who was three years younger than me, and considering that I'm barely drinking age that wasn't much. But he still handled his instrument, the electric bass, well enough. We started out well, until we got our first drummer. I'm going to be honest here: He totally sucked. He owned a Jordison signature drum set, which was a strange choice since he was most proficient at traditional Finnish dance beats. We carried on, and at some point got ourselves a new drummer. He was better, although a bit quiet and shy. At this point, our only "song" was a Sabaton cover. We tried composing our own songs, but the bassist wanted to make his own basslines. And he had zero knowledge of music theory. At some point I stopped going to our practice sessions, and eventually the band split up.
What did I learn from this? A whole lot of things. And I'll list them here.
First of all, the age. This is a problem mainly for young musicians. Age usually doesn't matter, but sometimes teenage bandmates can be annoying in the least, especially if you're almost an adult yourself. I'm not saying that you should discriminate on this basis, but if you're in high school and a thirteen year old asks you to form a band with him, don't hesitate denying the offer. If you're that age too, great, but if you and the only other bandmate have more than two years of age difference, you'll probably have a whole lot of difference in opinions.
Then the genre. We started as a melodic metal band, and as a prog fan I was thrilled with the opportunity to play some Amorphis. But things didn't go that way. Before I knew, our only song was the darned Sabaton cover, and I soon realized that the bassist wanted to make us what basically was a Sabaton tribute band. I suck at saying no to people, and I was too shy to propose my own ideas for cover songs. We never finished any other song than that, and the only other propositions were other Sabaton songs. Whatever you do, don't be afraid to express your musical tastes to the rest of the band. If you want to play a song, propose it. Discuss your genre with your mates, and be open about your opinion. You are a valuable part of the band, and you have a right to bring your own influence to it.
The drummer. The first drummer, I can't stress enough how bad he actually was. I'm not exaggerating here; He knew two basic beats and he couldn't keep up with the rhythm at all. I was okay with this since he was a friend of the bassist, so I really didn't have any say in the matter. But I should have had. If you think that someone is a weak link in the band, discuss it with the other bandmates. Your band can only be as good as the weakest member, so if someone really can't keep up and you're serious about your music you should discuss it thoroughly with your mates.
The main reason we didn't finish any of our own songs is that the bassist couldn't write the simplest line that fit any kind of theoretical okayness. This might sound unimportant; I can write the songs myself, right? No, if the bassist wants to be an active part of the composing. And this was so annoying, that I sincerely recommend you tell any bandmate who is like this to either learn his theory or drop the composing. If you want to be a serious band, you can't have that bad songwriting.
The new drummer didn't help much. He was quiet like me, so all the decisions were pretty much made by the bassist at this point. At a scenario like this, I suggest that you talk to the other quiet guy in the band and form a sort of an alliance. This way, you have a better chance of actually changing the course of your band. Just remember not to go too far, it might be annoying when a single person takes control of the band, but it isn't a good thing to be the only one left out of the management. Everyone should be an equal part of the decision making, so be sure to take everyone's opinion into account.
At last I just want to say that I should have quit the band. I just missed all the practice on purpose since I couldn't stand the band, and eventually it just dissolved. I wish now that I had quit in a proper way, to save a lot of frustration from the other band members. It would have left a better impression of me too. And this is a common problem with a lot of musicians. I often see people asking "should I leave my band" here on UG, and in my opinion if you don't like the band and can't change it for better, just leave. There's no point being in a project you don't enjoy or a project you might even hate. If you leave cleanly you'll save a lot of your own and your bands time, and you will not get bored with the thing you love as a musician; the music itself.
To conclude, the point of this article was to show you all the importance of expressing your opinion, using my own experiences as an example. I know a lot of people aren't as pathetic as me in social situations, but you still might face some of these problems individually. I'm not saying this is how you should act, but sometimes it's better to voice your opinion than to stay silent.