In Classic WoW, I progressed through Molten Core with the guild Fatalis Foedi, and participated in their first Ragnaros and Oneyxa kills. I was with them until that guild's leadership exploded - The GM transfered, taking with him the entire guild bank, and the quintessential "Main Tank / Raid Leader" disappeared under mysterious circumstancces - leaving the guild unable to complete the large majority of its content. This experience certainly shaped my views on guilds having designated "MTs" with loot priority, and overreliance on certain members - I made a promise to never be that tank.
I then raided with my current guild, LOKI, through BWL, and participated in their first Nefarion kill. I was with LOKI until my schedule changed and I was unable to make raid times, where I quietly stepped out. I had limited experience in AQ40 (the first 3 bosses) before Yakra was shelfed until the release of BC.
In BC, I started out in a guild called Dissention. As they progressed through Kara, they had a tank whom demeened every other potential tank, and did his best to be in the spotlight 100% of the time. My clashing experiences with him formed my strong opinions on the need for equality of tanks in a guild, and the sharing of the spotlight, all the way down to who tanks trash pulls.
When that guild blew up, I went to Mercenaries of Honor (MoH) - a highly casual, social guild which is among the oldest guilds on Darkspear. Despite my best efforts to get raiding going, guild policies prevented any type of gear/talent/playstyle critique, and non-existant standards for guild members prevented any type of progression - to this day, they are making passing attempts to clear Kara. I learned a great deal about attempting to lead raids before I left, and about the importance of raid members bringing 100% to the table. My attempts to educate their tanks was also the origin of Yakra's Theories on Tanking. (Despite MoH's shortcommings, I did meet some exelent players there, who eventually came over to LOKI with me)
After MoH, I teamed back up with LOKI, and have run the whole course with them, from Magtheridon to Illidan, participating in nearly every guild first kill (Exceptions being Maulgar/Gruul, VR, Rage, Antheridon, and Mother Shaz). Its by far the best run guild that I've been in, and is full of great players.