I felt powerful when my enemies started turning into piles of ash AND I have not even taken the "Disintegration" perk .-.

Yeah i feel pretty powerfull when that happens to, Mages are really powerfull i cant practicly one shot most dragons, and yet i still have to make sure i dont get done in by some guy with an axe.

Around level 35 I started to remove some of my 1-handed buff gear. I wear regular boots and gloves instead of armor now too. I want this game to remain a challenge.

It started in Helgen, really. First they put me in the same wagon as Ulfric, a powerful leader figure for many Nords. Then I was saved from execution by no less than a dragon. Obviously I had some clout...

After I spammed Muffle to get 100 Illusion and proceeded to Frenzy everything ;o

when sneaking allowed me to kill without being seen and i figured dual wielding is actually damn powerful with the right weapons and perks

Two words: Shadow Warrior!

When I can sneak in front of an enemy, go casually behind him, and slit his throat without him seeing me.

Around level 20-24 on Master I started worrying about the game being too easy and nothing being a challenge.

I had to put Intense Difficulty x4, Deadly Dragons and PISE on to try and add some challenge.

when i got dark brotherhood gloves and started slit the throats of most bosses

Dragon Armor, 100 Heavy, 100 Two-handed and Warhammer. Get off me.

I'm currently playing on Adept without the use of Enchantments or exploiting Smithing to its fullest. I still feel powerful at +35.

When i got the stagger perk with my Mage i stopped playing that character,I foolish kept playing for a while and lost a save point before i took the upgrade.

Too soon, that's the short answer.
And the long answer? It was very too soon.

About level 1

Being an archer, I felt pretty powerful from the get-go. Bleak falls barrow was a breeze. I could two shot everything that wasn't a deathlord. Those were three shots if I could maintain sneak. If not, yeah they could be a bit annoying.

This might sound lame, but when I killed my first dragon.

EDIT: Seriously around level 30 where my firebolts were taking down enemies without having to dance around too much or using shouts to by time then I felt more moral around level 35 when I get constantly one shotted by arrows but then around level 38 when I unlocked Dragon Hide the game felt much easier.

Yes another feat of skyrim that dont make sense. Your about to get your head chopped of and lo and behold a dragon comes to save the day and whats your reaction? from now on all you do is kill dragons nonstop ...
Thats gratitude for ya right there.

As soon as I realized how powerful my (Legendary) smith-altered gear was.

That, and having multiple perks in a variety of different talent trees. At max level, you could easily have a bunch of perks in almost all the talent trees. I have yet to make it to max level, though.

Yeah, good point. Probably the first solo dragon kill.

"Feel Powerful" =/= nothing challenges you anymore, or nothing can kill you. I still had things giving me a good fight right up into the upper 40's. But there were still plenty of moments that I felt badass before that.

Probably with a maxed out sneak tree and killing an enemy with multiple sneak attacks without them even having the chance to turn round.

Around level 30, I reckon. Basically, as a mage, that's when I started enchanting my gear to reduce casting costs.

I started to feel powerful for the first time when I killed my first non-scripted dragon(which was my second) without any trouble. I was quickly put back in line by a giant(I was only at level 9 or so).

Now that I know the game a bit better I start feeling reasonably powerful when I get some of the "core" perks of my planned build, usually around level 20-30. I feel pretty powerful on my current character(level 39), running around on expert with an exsquisit iron warhammer(custom enchanted, but no charge left) and epic steel plate armor(with only one part enchanted in a way that helps me and that is 25% fire resist). Makes clashing with boss-type enemies really fun.

Around 25 when I killed pretty much everything in a hit and was completely undetectable. My biggest complaint is the fact that enemies are almost always by themselves. Most have their backs to you or ar sleeping or sitting down. Then you have Draugrs that climb slowly out of a crypt or coffin, or Wolves/Bears that let you know they are coming or nearby well before you see them. It is way too easy to get the drop on enemies. It would be a lot more challenging and fun for me if there were much larger encounters more frequently, and if single targets were actively searching or pathing (and/or) there were ambushes that would occur. As it is now, becaues of how encounters are strucutred, it is really, really, really easy.