Huzzah kind of touched on this, I’m going into more detail.  The answer is, without a shadow of a doubt, is Shadow/Disc until you hit at least 55.

Why you ask (or you ask only if you haven’t mentioned priesting around anyone who has ever played a priest and isn’t a moron)? Until the end game instances (that’s Scholo, Strat, LBRS 5-man and UBRS) a shadow priest can successfully heal a group with no issues.  Don’t get me wrong, they CAN heal a group afterwards, but it is more difficult. So, a shadow priest can perform the healers task no issue until end game all the while becoming the highest DPS dealing class in the game. Sounds like a no-brainer?  You’d think that, but then, if you do, you haven’t been to the priests forum. To conclude…if you are not at least level 55, and your priest is not shadow specc’d, you are missing something.</p>

However, I have known people in large guilds who stayed Holy/Disc their entire leveling and played strictly in groups in instances.  I think this could be effective, since instance experience is really nice, but the limitation of constantly being forced to be in groups very much limts a players choices

So how do you build it, especially since the Disc part of the build is as important as the shadow part.  Basically, along the priest talent line there are five major talents and the order you get them in matters little in the long run.  Each of these significantly improves the soloing  capabilities of your priest.

A priest works his way down the shadow talent tree towards shadow form and at some point, drops eight points in Disc.  You should only drop eight in discipline until you have SF.  After that, drop the rest in discipline.  This is the best way to level your priest.

When to get Improved PW:S is a tough decision because while it significantly increases your survivability, but pushes back your acquisition of Shadow Form eight 8 levels.  My suggestion is to pick up mind flay, then pick up Improved PW:S.  When you hit level 41, respecc to pick up Shadow Form and start working your way down the discipline tree again.

When you hit 55 (though I strongly suggest waiting till 60), respecing to holy is a smart move, but not the only choice. There are several valid major builds, and many variations of these builds. I’m going to briefly talk about each, highlighted the key aspects. As you decide on your respecc, remember that each point you don’t put in shadow further restricts your ability to solo. Also remember that because of the diversity of the priest talent tree, there are many variations of each major build that can tweak your build.

In addition I suggest starting to do almost exclusively instances once you hit 55. The experience is very good, and you start to acquire equipment that you will hold on to for a while. If you wait until you are 60 to start doing end game instances, you will still have to spend the same amount of time in Scholomance, Stratholme, and Black Rock Spire...you just won't get experience.