If every person in a group plays the same class and uses the same skills, your group will fail.
If you have good healers (and players), your group will usually need (total members /4) healers. If that doesn’t sound right, take the total number of people in group (round up to the nearest multiple of 4), then divide that by 4. For a group of 12 people, you would need 3 healers (if they are good and your players play well). If your players are not performing and simply stand in damage, then no amount of healers will help you.
A dedicated person or two on cc (to fear, talons, etc.) is important. One player focusing on heal debuffs (Fasala’s Guile for minor and Corrupting Pollen Warden skill for major) is also very important.
A few people on siege is important too. Cold Fire siege does a reasonable amount of damage to players (even more in non-CP). Stone trebuchets will hit for about 10k damage. That’s almost half of most opponents health (or the full value of most shields). One or two people hitting with stone trebs (or even cold stone trebs) at the start of a fight can make for a very quick win!
A few people running particular skills (such as efficient purge, siege shield, etc) is very important, as are retreating manoeuvres.
A diverse assortment of ultimates and ordering them such that the group leader can call for them is critical.
When forming a group (or adding people to a group), spending a few minutes making sure that the group composition makes sense, key skills are available, and ultimate orders and calls are understood by everyone is critical.
Please read through the Raid Requirements Document for more information on group composition, builds, and details.