Here are some thoughts on how to build a raid, including some personal preferences. These are listed in priority order.
1) Raid leader. This is not necessarily just one person. For example, my current guild master has an amazing raid background but doesn't know WotLK bosses very well. It makes sense for us to have two leaders, him for his great raiding experience and me or some other officers who knows the WotLK bosses. Also make sure you have a reliable person to collect loot. You can't lead a raid very well if you're hunting through vast piles of dead bodies for junk greens and occasional BOE epics.
2) Main tank. Some people doubt that there is a main tank but Veneratio explains the concept of the main tank in the modern raid setting wonderfully well in his recent podcast
The gist is twofold: in a tank "adequate" or even "fine" is an unacceptably low standard to aspire to and the main tank is the tank everyone else wants to see up the front tanking the big boss. This will change almost certainly as your guild grows and changes and it's the job of a good raid builder to have his or her finger on the guild's pulse in terms of who everyone wants to see up front tanking. If this person has dual spec they may well be double tank specs.
3) Additional tanks. Depending whether it's a 25 or a 10 man raid and depending which bosses you expect to try you will probably need more tanks who won't tank all the time. These tanks should be capable of doing good dps either because they have a dual spec of tank and dps specs or because they are tank classes that can hit hard even when tank specced (death knight and feral druid). You may be able to get away with dps specced death knights or ferals in these spots if you lack players who want to tank.
4) Full-time healers. For a 10 man you need 1-2 full-time healers and for a 25 man you need about 3 full-time healers. This is low and the reason it's low is because of dual spec. You really don't need to cart 8 bored healers through trash when they have little to do. For the most effective raiding don't have people healing when there's not enough for them to heal. It's boring and you are wasting slots that could be used for dps.
5) Dual spec healer/dps. For a 10 man you're looking to bring your total number of potential healers to 3. For a 25 man you're looking to bring your total number of healers to about 8, possibly 9 for some Ulduar bosses, I honestly don't know. The modern raid game is pretty unforgiving of the tactic of healer stacking. It won't help you kill Sartharion 3 drakes, it won't help you kill Malygos because you give up too many dps spots.
6) Stack buffs. The next stage is to stack raid-wide buffs. It's important to remember that your team so far includes some dual speccers so only count their buffs if they bring their buff in each of their spec. (You don't want the raid to lose Windfury when your dual spec shaman has to heal). Use this tool to calculate:
Also you may want more than one source of Replenishment. This buff is so important that uptime needs to be close to 100%.
7) Fill the raid. Your guild will have its own policies on how to fill a raid. For progress-minded guilds any remaining spots should always go to your-hardest hitting dpsers. Even on farm content these players should get first refusal on raid spots because improving them helps your raid more than improving a guy who'll be benched for tough content. You want a combination of great dps meter placement and great survival skills from these players.
8) Finally, note any recruitment issues. If you were short of players at any of these stages evaluate whether this is a temporary aberration or an ongoing problem. Perhaps none of your usual high attending tanks showed up for a night. Perhaps you forgot it was Valentine's Night and have now discovered the hard way that all your Druids are smooth-talking slickers who have opted to forego raiding in pursuit of more stimulating games. If however you are regularly short of some role or buff you need then recruit.
Wondering what comes next
23 hours ago