10 mans - great if you have a close group of friends. Problem is that if 1 person takes off, you typically have a larger set back. Any major guild drama often results in needing to replenish the ranks or guild being done. Fights are adequately set up to compensate for fewer ppl's rng for messing up, but fights drag on a bit
20 mans - imo the best balanced. The bosses health and length of fights seem to scale nicely with 20 ppl. Healthy balance between classes and ppl in raid/guild.
25 mans - something has always seemed off on the balance. Allows for a couple players to not be perfect on execution or get a carry or 2 in safely. Guild dynamic tends to be similar to 20 mans. However, in todays wow, filling a 25 man is a bit more shakey on occassion, but we aren't a high pop world with a queue anymore.
40 mans - holy cow - if there wasn't drama, then you weren't in high enough progression it seemed. So many people gave rise to so many cliques. If you weren't present, you could actually be totally forgotten. Allowed for multiple ppl to get carried through or do horrible. Especially back when the originality of fights and their mechanics were so high, it was a recipe for a tough progression road. Hard to keep that many ppl happy, and on the same page. However, it promoted great class unity, since you had to work with the other 5 members of the same class to pull off the victory (yes before we had shamans and dks).
In short, 20 mans were nice, 40 mans were lolz worthy, but I do miss the class unity and being able to see 5 ppl dead and it being ok and not looking like a wipe.