My paladin Liberais and Lori's druid Amedina got up to about level 42, then one of our frequent teammates (Marragan) got very busy. We spent two or three weeks wondering whether to wait for him or outlevel him. We finally asked him what we should do, and he told us that he was going to be busy for quite some time, and we should go ahead and proceed without him.
Marragan was a warrior, and a frequent tank for our group; my paladin had a half-tanking/half-holy build, but was tanking much more often than he was healing. So with us planning to leave that warrior behind, it was a good opportunity for me to switch to my warrior character as Amedina's teammate; my warrior Roktazza is specced for tanking, and was of almost the same level as Amedina. Liberais can go back to healing another time--I do not need three high-level tank characters.
Switching from Liberais to Roktazza has been a delight. It's much easier to find gear, and I feel as if I'm killing things more quickly and taking much less damage doing so. And most importantly, I'm not running out of mana all the time.
I have realized that I really hate playing a character who is chronically short on mana. A fair bit of this is that I tend to treat water as a scarce resource, and I only drink if I need to, because I fear running out. (I know, it's silly of me.) But I also really dislike the interruption of drinking or waiting to regain mana--I feel ready to go on to the next fight, but my character is not.
I've had a lot of trouble finding gear for Liberais that would help with his mana. And the claim that a paladin tank gets all the mana they need from being healed has not been true for me up through the low 40s. So playing Liberais has been a constant frustration of fight, run out of mana, wait just long enough to get just enough mana for the next pull, repeat. I can reduce the mana costs of a fight significantly by using Seal of Wisdom--but that means that I'm not using damage-boosting abilities, and my killing pace is glacial.
(This hatred of being chronically OOM also goes a long way to explain why I lost interest in my enhancement/elemental shaman--he also didn't have gear to give him enough mana to use effectively.)
Roktazza and Amedina have now finished Zul'farrak at last. (I hate the respawn there. On two previous tries, we had had fair success until we wiped and decided it it wasn't worth the bother of clearing through the respawn again.) Miss Rok is 47 now, Amedina is 48.
I'm not sure whether we'll make it to Maraudon--we're close to outleveling it already, and it's difficult for us to find the time for a long instance like Maraudon, and difficult for us to fill a group at that level. But I'm considering plans for expediting it. Several higher-level guildmates still have the Scepter of Celebras, so we could use to skip the first part of the dungeon, and have a warlock summon in a replacement for the higher-leveled guildmate.