I'm going to lay out my current Shadow talent build and explain why I picked the talents I did. Keep in mind that this spec is not for every aspect of the game. It's not even for every player. I'm using this spec as a level 70, PvE raiding spec. It's not perfect, I'm not completely happy with it, but I think that's because our trees are currently meant for level 80 and not level 70. I haven't been able to come up with something I like better yet. When I plan ahead for level 80, it feels much more complete. We don't have the luxury of those extra 10 talent points yet, so this is what I'm using for now:
I skipped Blackout because its raiding applications are very limited. It works on some trash, but never bosses. That's a big 'meh' for sure. I went with Spirit Tap and Improved Spirit Tap, because that's going to act as a Shadow Priest's Meditation for now. Finally, Tier 1 of the Shadow tree has something genuinely useful to raiders!
My first choices in this tier were definitely Improved Shadow Word: Pain and Shadow Focus. They're very clearly good talents, and I don't think you can get away with skipping these for raiding.
You'll notice I only put one point in Shadow Affinity. Originally, I was not going to put any points here. Save this talent point for something else, if you want. You're probably thinking I'm absolutely crazy for skipping a -threat talent as a Shadow Priest, but threat is actually not really an issue for us currently. Tank threat is through the roof, and right now a good tank will be able to hold threat off of you in even the most threat sensitive encounters. If you want to, you can shift some points around to fill this out, but I don't see a need to do so at level 70.
If you are a Shadow Priest and do not take Mind Flay you are a fool. This is not up for debate.
I filled out Improved Mind Blast, all 5 points, because... hey. More Mind Blast. There are arguments for only putting four points into the talent, or even just three points, but I personally like going all in with it. Even if I don't end up using it as soon as it cools down every time it does so, having it cooled down more often is something I find beneficial in the long run.
I skipped Psychic Scream, and thus Silence, because the two just don't have enough benefit in raid dungeons.
This tier is one of those very, very obvious tiers. For PvE, it's all good. Lots of mana regen via lower Shadowfiend cooldown. More range, which is always nice. Shadow Weaving is fantastic, even if it's only personal damage now. Just fill out this tier.
Vampiric Embrace was my first choice. That's another must-have for any Shadow Priest, considering every other essential Shadow spell is tied to it. I didn't pick up Improved Vampiric Embrace, because at 70 you don't have enough talent points to get everything you want, and this felt like something I could cut without feeling bad. When the Vampiric Embrace nerf (which was needed, but perhaps too much) goes in, I may reconsider these points, I might not.
Focused Mind was an easy choice. An extra 15% mana cost reduction on our 'nukes' seems like something everyone would want.
I completely filled out Tier 6. It's all good stuff. More damage, more crit.
Pick up Shadowform. That's an easy choice. Fill out Shadow Power, too. Shadowform is essential and more damage on crits is an easy choice, really.
Fill it out. Improved Shadowform is fantastic. Misery is great for raids and personal DPS. I don't think I need to explain further.
I skipped Psychic Horror, that's definitely a PvP thing, or maybe a soloing thing. Not so much a raid talent. Nab Vampiric Touch, fill out Pain and Suffering. Pain and Suffering will essentially remove Shadow Word: Pain from your damage rotation. You'll hit it just once at the beginning of a boss fight (it'll drop sometimes, phase changes and such, but you get what I mean) and when you would've hit Shadow Word: Pain before Patch 3.0.2, you'll be hitting Devouring Plague instead.
You only have one option here. Twisted Faith. It's a good option. Fill it out.
Dispersion is optional, but I took it because it's new and exciting and when I push the button and turn into a purple cloud my guildmates exclaim, "WTF was that!" The mana regen has its uses for sure, but I haven't seen it really pay off yet at level 70 in PvE. If you wanted points in Improved Vampiric Embrace or another point in Shadow Affinity, this is one of the points I would move.
We have a mind-blowing 5 talent points leftover after this. The Shadow tree is absolutely loaded with good talents. I wouldn't call it bloated, because it all works together quite nicely, but at 70 it really doesn't leave much room to branch into other trees. No big deal, though. Those 5 talent points fit nicely into Discipline.
Twin Disciplines is where your points will go. It works on about half of your damage spells, so why not? If I remember correctly, it was changed so Mind Flay is modified by this talent as well. It's a solid choice, I think.
And that's my current talent spec. 5/0/56. Is it perfect? No, I don't think so, but like I mentioned up above: I don't think a 'perfect' spec can be achieved until level 80. That's sort of the point of the expansion. I think what I have is as good as I'll get as far as my wants and needs for now.
I definitely encourage other Shadow Priests to show us their builds in the comments section below. I'm definitely a raiding kind of guy, and I approach my talents that way. If you have soloing builds or PvP builds you like, show them off