Yes, leveling does get quite a bit easier, but not till you have 20ish points in the tree. It gets especially easy with Shadowform at level 40.
Also, blackout is a no no for leveling. It has very minimal usage for leveling, interrupting the damage of mobs for 3 seconds, and maybe letting you get off another mind flay. Also, it seems to stun mobs when they are out of range of your mind flay, making you waste the 3 seconds by running closer to them.
So here's IMO the BEST leveling spec pre-40.
10-14: 5/5 Wand Specialization - get a good wand, search the AH for the lesser and then greater magic wands, and try to upgrade it every 4-5 levels.
15-19 5/5 Spirit Tap - get it, stack spirit gear as much as possible, but make sure to maintain enough stamina and intellect so you don't run out of mana or die too quickly
20-21 2/2 Imp. Shadow Word: Pain - get this, pre-mind flay and even pre-Shadowform, the way that I leveled was cast, Mind Blast, then SW: Pain, then wand the mob to death. This gives you two free ticks of SD: Pain for no extra mana
22-24 3/5 Shadow Focus - slightly a "filler" talent until later on in the game, but does up your DPS and allow you to take on mobs 3-4 levels higher than you with relative ease
25 1/1 Mind Flay - at this point, MF does not deal that much damage, but still get it. I found myself still using the MB/SW:P, wand rotation most of the time, but I'd use MF a lot on runner mobs or times when I was in a group
26-29 4/5 Imp. MB - this ups your DPS incredibly if you're using MB every time it's off CD, but is slightly a filler talent until later on
30-32 3/5 Shadow Weaving - I find that a 60% chance of upping your damage was plenty while leveling, and didn't bother getting 5/5 till around level 55 I think. Some people say get Shadow Reach first, and either talent is a good choice
33-34 2/2 Shadow Reach - makes MF a lot more useful by allowing you to get ~ 2 full MF's in before the mob gets to you, as long as you are moving backwards during the Global CD of SD: P
35 Vampiric Embrace - get it, not only is it a stepping stone to Shadowform, but makes it so you basically never have to bandage/eat/heal. Just cast it on a mob every 3-4 fights and you'll be back at around full health.
36-37 Imp. VE - get this as well, just makes one of the best Shadow tree spells even better, especially in groups. (Later, when raiding, not as useful)
38-39 5/5 Shadow Weaving or 2/3 Focused Mind - your choice really, by this point it won't really make a difference
Level 40 (YAY!!!!) respec to http://www.wowhead.com/?talent=bZZxMhpMxR0t
I cannot stress enough how important it is to have the best wand possible pre-40. A good wand and with wand spec. can and WILL give you more DPS than the majority of your shadow spells. Also, if you wand once you've cast MB and SW:D, you'll be regenerating mana before the fight is even over, as well as getting spirit tap once the mobs is dead. If you stack tons of spirit, you will rarely have to drink, and once you get VE you barely ever have to stop killing things.
I know leveling as a SP is a bit difficult until you get shadowform, but just stick it out and you become one of the best classes to level with. Also, it's kind of a general rule that any class is difficult to level with until you have 15-20 points in a talent tree. Enjoy, and welcome to the wonderful world of Shadow Priests!
Edited, Dec 15th 2007 2:01pm by Schmere