Regrinding guild rep doesn’t seem quite as onerous as people think, unless you just hate dailies [and I know some people are of that mind and hey, it's cool - we all gotta play our own ways!]. But there are a number of ways to regain guild reputation. In fact, you can easily be exalted with your new guild in about 4-5 weeks, more if you’re more casual about raids/dailies. Either of the BOA tabards will enhance rep gained per boss/instance/daily. You can either bring it with you or gain it at Friendly [blue tabard for 50% gains] or Honored [purple tabard for 100% rep gains].
1. Raids – Guild raids give quite a bit of rep. Hubby and I have been attending our new guild’s farm night where they blaze through T11 raids. Alts are welcome, but we’ve honestly been glad to just play our mains and gain the Valor points and Guild rep that comes with it.
2. Guild Dungeons. Five mans with three or more guildies will give Guild Rep [more with more guildies]. We’ve managed to catch a few of these as well and it’s been really profitable reputation-wise.
3. Daily Quests – While one might think that the Molten Front dailies would be ideal, most of the dailies actually inside the Molten Front are unaffected by the Guild Tabards. So while the outside quests do provide double rep with the tabard, and are worth doing, I’d skip the inside ones unless you aren’t finished opening everything up. The Tol Barad quests are actually very good for guild rep as all of them become double rep with the tabard.
EDIT: Apparently zoning into the Molten Front deactivates the tabard and if you remove it and re-equip it, the tabard gains will work appropriately.