I primarily played Horde during Vanilla. When TBC beta started, I saw videos of shockadins in arena, unleashing 1.5k holy shock and 800+ judgement of righteousness crits. This was back when 4k health was a lot, so 2 holy pallies together could quite feasibly kill somebody instantly if everything crit.
I levelled 1 to ~50 at ret, but once I got to some of the redesigned SP plate (designed for ret mostly, I think) in BRD and LBRS etc, I respecced, and levelled to 70 as holy. In hindsight it was a huge mistake (and this was before there was any interaction between +healing and +dmg, too), but it gave me a decent understanding of the basic mechanics of healing as a Pally. In the end though, it only really takes 1 or maybe 2 at most instances to get the hang of healing, and it took me much longer than the 1-2 hours they would have taken to hit 70 as holy instead of ret.
Regarding the healer shortage, it's something I've seen a fair bit on Frostmourne-US, especially when I'm prot. That said, it's something I quite like in the end, because it means I can be quite picky about choosing groups. Farming badges for heirloom items which will get 1-2 hours of use a month as I contemplate levelling alts then abandon the idea has never been easier.
I really enjoy being able to just ignore people who spam invite me despite the "PST before inv" in my comment, or those who PST asking what my SP is, then tell me that ~1750 (I have an iLvL 200 weapon and no SP trinkets, and gem purely for int) isn't enough for heroics (even though it's enough for me to have solo healed 10 Maly, 25 Sarth, and most 10 Naxx fights where it's physically possible (Thaddius / 4h / Gothik etc can't be done)).
If I were being objective, I'd say that it's probably turned me into an arrogant prick, but then again I don't have to be objective, I'm a goddamn healer!
I have no problem levelling as a holy pally. But it seems like a lot of people make fun of me for it. People love to ask why I don't level as ret.
I've gotten this as well, but I've loved my holy pally. Holy Shock down to a 6 sec cooldown; Judgments of the Pure + Enlightened Judgments= more win for me, I get a lot of dpsers pissed at me when I'm doing dailies and I can steal mobs because of the range that I can get compared to other classes.