Very happy

I could not be happier about my boosted 90. I love her. It is letting me experience things that I would probably never accomplish. My only previous 90 was a Monk, who I enjoyed leveling, but hated end-game. I just didn't enjoy her. I decided to use my boosted 90 on a Shadow Priest, which was a class I had been enjoying leveling, but was having difficulty questing with. My highest Shadow Priest was about 66 before I finally succumbed to the Outlands rut.

My new Shadow Priest plays in the way that I expected, and I am not feeling overwhelmed like I thought I would.

But anyway, on to why I'm so happy!

I just soloed Onyxia! I've been wanting to do this for a long time, since the Onyxian Drake is one of my favorite mounts in the game, and I've wanted to grind for it. I had tried to with my Monk, but it just wasn't working for me. I just did it, and I actually enjoyed utilizing my character to the fullest.

And just to prove that I indeed did know what I was doing:

Made sure that I had Power Word: Fortitude and Inner Fire up before entering the dungeon.
Cleared the trash. Had Power Word: Shield up when I could. Vampiric Touch > Shadow Word: Pain, Mind Blast > Mind Sear Mind Sear Mind Sear > Mind Blast. Repeat unless I had 3 Shadow Orbs or Divine Insight proc'd.

First stage of Onyxia wasn't bad. Mostly my usual rotation + Power Word: Shield + Renew.
Second stage was a little trickier. Used Mind Sear on the trash. Kept using my rotation on Onyxia. I'm pretty sure I used Vampiric Embrace here.
Third stage was easy, spammed my rotation. I'm pretty sure I used Mindbender and Fear Ward as well. Didn't run low enough on mana that I needed Dispersion.

Throughout the entire fight I kept Power Word: Shield up whenever it was available and used Renew if I had taken hits before.

I want to learn to utilize Silence and Inner Will more, but I'm pretty happy with myself otherwise.

Just a question though: Does Mind Spike have a purpose? I mean, it says that Mind Blast does 52,986 damage and Mind Flay does 42,849, but Mind Spike only does 35,505? Is it because it does "shadowfrost" damage that it has a purpose? Because currently I don't see the purpose in using something that does lower damage and gets rid of my DoTs. Am I just missing something totally obvious here?
Oh, and gear!
Most of what I was using was just the gear that was provided with my boost. Although I do have three pieces 496 pieces from Timeless Isle (Amaranthine Robe of the Wavecrest, Amaranthine Handwraps of the Decimator, Amaranthine Cord of the Impatient). Slowly working on Timeless Isle to get gear.

My eventual goal is to learn how to DPS in dungeon/raid setting. I've only ever healed through dungeons, and not even high level ones. I really do love playing a Shadow Priest though, and would love to learn one. Pretty lofty goal for me though.
First step I think is to solo some stuff, learn basic mechanics such as silencing, maximizing DPS, etc. After that, I'll work on two-manning some raids with my dad (much more experienced in raid settings than I am), so he can explain the things I'm missing.
After that, group up for scenarios. Then dungeons. Eventually raids, although that probably won't be until the next expansion...
I want to be confident in my skills. I don't want to doubt myself the entire time because then it will start being fun. By the time I'm entering raids I want to be indistinguishable from a non-boosted character. I don't want to go in and have it be obvious that I didn't play this character from 1-90.

Good plan? :)