I've been having a lot of fun taking my priest through LFG instead of questing. I begun my LFG journey at level 12 and i'm level 50 now after 46 instance runs. Not bad and a lot more fun than the questing grind for me at least.
However I'm a little concerned about when my character hits the various 'gear jump' stages, such as BC, Wrath and Cata. Being purely LFG, my only gear upgrades come from LFG drops/bags. I'm concerned that when I start running the expansions content, for a little while my gear will be quite low considering the quest gear available in those new areas has a big increase in stats. Yes gear will drop for me, but I'm worried for the first few instances I will be quiet down on stats as I won't be wearing any of that quest gear.
Am I right to be concerned? Do I need to explore using tailoring to fill in the gap, or should I have confidence in my LFG blues?
I wouldn't worry too much about it. When first hitting Outland/Northrend, the first dungeon might be slightly more challenging, but it would even out pretty soon. You should be used to healing, so it might be challenging, but never overly hard.
Lowlevel dungeons (and by lowlevel I mean anything non-heroic, sub 85) are tuned so low in difficulty that you really won't notice the gear jumps at all. In other words: Don't worry about it.
You might start feeling getting some OOM-issues around lvl 84, which is the point where the mana costs of your spells suddenly jumps up while your gear doesn't improve all that much. There's nothing to do about that other than just making sure your tank pays attention to your mana and gearing up as much as you can (which you should probably do by LFG dungeons anyway).