Originally Posted by Dite
You also have to remember that you don't level up by the quests you do, but the skills you use. So, if you go around the whole game using Sword & Shield, and using Light Armor, you'll once you level them all, you can probably do a whole questline without leveling up. Depending on how you like to play (you like one or two things or like to change up how you fight), you may have to go out of your way to level up.
Getting to level 50 was the last achievement for me. I used what I said above, and used Archery for long range. I didn't pickpocket, use much magic (some restoration, but not much), or use two-handed weapons or heavy armor much. And I generally used what I found. I created one potion, one enchanted item, and one smithed item early for the achievement, but afterwards, I barely touched them. My focus was the questlines and I played how I liked.
After all my questlines were done, my light armor, sneak, lockpicking, and speech were 100, and my archery, one-handed, and block were pretty high. But, everything was pretty low, and I wasn't even level 50.