To the topic: there is always that fear. Just like there is a fear being a healer and DPS (DD - whatever we are calling them). With any pick up group, there is a chance you play with people who are learning or just not grasping the job they are doing, whether it be not healing enough, not keeping hate, or not regulating the dps. I take those parties as higher learning curve parties - stressful, but they really push you in your own role in the party. Provides the opportunity to suggest a little friendly advice on how the group can operate better by doing XYZ is only a chat box away. Do it a few times, and you get over the fear of rocking the boat. Just be kind and if the rebuff is trollish, ignore them. If they can't take it as something to try or something they want to do, then so be it - you can't do it all no matter how much you want to. If it really is not working out and you are repeatedly dying, there is no shame in bowing out - just let them know you are leaving before you dart off, common courtesy and all.
But those parties aside, sometimes you get in a really great party that flows and it is like watching the plot of a movie unfold seamlessly. Those are the ones you need to make yourself fully aware of and live it up then and there. They are what drive you on - always looking for that perfect synergy.
Most of the time though, you get a really good player or two with the others just there to push buttons - add the ones that compliment your gameplay style to your friendlist and go from there. Those parties are not horrible, but not ideal either - however they make up the bulk of your grouping in pick ups. Find a great tank - F-list him/her. Healer just saved the entire group with an amazing heal rotation, added. DPS that unleashed the apocalypse just as the mob was going to dust the party with an AoE? yep. And after all this happens, you suddenly don't fear pick up groups as much anymore because they are a kinda metagame to see how many great players you can add to your friendlist.
Just my opinion and YMMV, but it works in every MMO - to an extent.