Which lvls takes the most time..

[Lackluster]

Most definately this. 1-40 can be a real pain for priests (not that it is difficult just boring and slow) but once you get shadowform you should move a lot faster.

I would have to disagree, holy fire and glyphed smite can totally destroy mobs at lower levels. Why bother with dots when you can just as easily 3-shot mobs?
The only advantage to dots would be multi-dotting, however you don't get a lot of use out of spirit tap when many mobs die at the same time and I noticed I went out of mana very quickly when trying to multidot as a lower level shadow priest.

[Vanch]

The levels where you post in the wrong forum, sadly.

About as much wit as a flick to the teeth.

[Darios]

40-60 tend to be the worst for me, have to run all over the damn planet

[Etori]

For me it's always level 70-71. God I hate those levels, but that's just a matter of like and dislike. Once I'm past Howling Fjord & Borean Tundra, I'm all good.

As for shadowpriest levelling, it's probably the time before you hit level 40 and get your Shadowform.

[Shazzia]

50-58... where the fuck am I going to quest?
70-80 is faceroll

[Axaruz0]

1-80 was the worst one for me

[Siight]

I haven't tried a priest recently, but back when I found mana kind of sucked for them, I then found using a wand as well to help kill everything made mana not so much of an issue.

Longest levels I found are when you hit northrend...70+. Just feels like it takes forever to gain a level.

[LQDMTL]

1 to 15 I did in a single evening back in Vanilla wow on my first toon (a rogue). I didn't find it very frustrating at all. I'd say 1 to 15 in Mulgore is absolutely horrendous, though. But Alliance 1 to 15 (especially Nelfs and Spacegoats) is relatively painless.

15 to 21ish isn't bad either. Especially if you're alliance and just head to Darkshore. Very fast quests there, up until around 19ish. Then just hit Ashenvale for a bit of smooth sailing, or Redridge.

22 to 38 I never had a problem with. Redridge, Wetlands, Duskwood, Southshore and the first half of STV, all great. Even back in the day (vanilla wow, with no mount). Now with a mount, much faster exp and heirlooms... pfft.

39 to 43 can be atrocious. You seem to travel more than quest and the quests are either mostly below you or mostly above you, with many of the quests just being downright tedious. I have never, in 6 trips through that range so far, enjoyed that part of the leveling journey. Never. And I'm in the middle of it again as I work on my 7th 80.

43 to 50 seems rather easy, even if you do a lot of running (F U, gadgetzan and your stupidly expansive desert).

50 to 53 can be a bit boring, what with all of the travel and... well, I hate Ungoro.

Then 53 to 58 I find rather easy. It slows down a bit at 57 near the middle/end of questing in EPL, but the WPL chain is fun and very quick if you stack the quests right.

58 to 68 I've done in just a couple of days, so I wouldn't say it was painful. It wasn't very fun my first trip through, mostly because that toon failed. My second trip through I lived in dungeons as much as possible and had great, blue-quality gear in nearly every slot from 60 all the way to 70. I loved it.

When LK launched I took a DK up as my first 80. Went from 68 to 70 in a day, made a level and a half a day after that until 78 then it slowed to a single level per day. Around 6 or 7 days, or thereabouts. Personally I think Northrend is the best leveling experience of the bunch.

Hell, with Heirloom items and rested bonus (I went on vacation for a week and a half, I didn't intentionally camp the toon) I went from fresh 77 to 80 in a single day this past Friday on my hunter (and didn't touch a dungeon at all during). I just don't see where the complaints come from.

EDIT:
I had a tough time from 15ish to around 25ish on my shaman, when I leveled her. She just seemed so squishy, and mana sucked so bad.

[Fetacheese]

1-10 Takes about two hours
10-20 Usually fast, but priest mana is hell
20-30 Usually very fast, but priest mana is terribad at this point
30-40 Usually sucks, but priest mana is better by now
40-52 Speeds up a bit, priest becomes much more powerful
52-58 WTFSLOW
58-60 Should take you a few minutes
61-68 Outland, slows down a lot but is still awesome
68-70 zomgdast!
70-80 Slowest, but you definitely feel like you're leveling.

[Aphie]

50-58, by far, for most classes. EPL/WPL are the most soul destroying places to level ever, I've never done it by myself apart form my first character, i've always asked for a RL friend to boost me in scholo and such, and in return give them boosts ofc. but other than that, everything else i find really, really easy to do.

[Takanami]

1 to 15 are the most hard/frustrating in the game for all classes. Longest time to lvl are the 70+ range.

Honorable mention - the few lvls between you and the dark portal


this is very off-topic though

This is the best answer really. From 15-40 You can quest and do dungeons with one spec, shadow can heal and disc and holy can actually dps fairly well. Then DS at 40. So all and all its a total breeze till 70, then it just gets tedious. However, 55-58, I'd say those are just as easy, it just seems longer cause your on that brink to so much more exp and munnies. :P

[MisterSoup]

Really, it depends on whether or not you'll be getting boosted through instances by a high-level, running instances using the random dungeon finder tool, PvP, or questing.

I've leveled 8 characters so far, and generally the levels between 40-48 have always been the most grueling for me. Here's a few tricks I've picked up along the way:

Getting Boosted
10-15 - Ragefire CHasm
15-20 - Shadowfang Keep
20-35 - Scarlet Monastery
35-45 - Zul'Farrak
45-58 - Stratholme

PvP
While the amount of experience awarded from Battlegrounds is in fact nerfed from when it was previously introduced (and grinding Alterac Valley was almost too good), the first random battleground you complete is worth the experience of a dungeon quest, as well as the experience you'd normally get from the battleground. Do this every day!

Reaching Outlands
Especially with any character that can heal themselves (or has hefty mana restoration), Outlands takes no more than about a day to complete. Doing every quest available in Hellfire Peninsula will take you from 58 to 62 (with Heirloom gear, of course), Zangarmarsh will take you from 62 to about 65, and Nagrand will get you to 68, if not just under it.

Keeping yourself queued at all times for a random Outlands dungeon will speed up this process.

Hostile Servers
If you are playing on a PvP-enabled realm, you're going to get ganked. A lot. An easy way to avoid this as much as possible is to simply stay off of the roads if you suspect that any flagged high-levels are nearby.

When you do get ganked, you might feel like hopping on one of your own high levels, or trying to have a friend come out and kill the other person for you. Don't. You'll only waste loads of time, piss off the high-levels in the area, and possibly spend even more downtime than you had intended to spend.

GOLD AND PROFESSION!
Level up your professions as you are leveling. The temptation to simply buy all of the mats needed from the Auction House can be way too great if you wait 'til level 80 to max professions. Do them yourself, and for free, and you'll save anywhere from 2-6k per profession.

Especially with full BoA gear, you should never, ever, EVER buy anything other than food and a glyph or two while you are leveling your character.

Sell all of the quest rewards you get, put any BoE you find on the auction house (or just vendor it if you're too lazy), and again - DON'T BUY ANYTHING. Leveling a character from 1-80 yields about 7k gold (give or take), including having bought all riding skills, as well as Cold Weather Flying. Not a bad chunk of change to get a fresh level 80 started!

[Laukkanen]

As I rarely ever level dps classes now I'd say the 50-58 is the most painful bit. No good dungeons to do imo so just quests. Or well, BRD is decent.