I recently hit level 20 in League of Legends. Level 20 is probably the second most important level to hit with only levels 21 and 30 being even anywhere near as relevant. Why, you may ask? Well, every level gets you a talent point and a rune slot. Talents are free and can be changed around at will at any point you aren't actively playing a game. It takes 21 points to reach the bottom of a talent tree and the 21 point talents in 2 of the trees at least are incredible. (The three 21-point talents are 5% extra base damage, 4% less damage taken, and 15% cooldown reduction on your 'summoner' spells.) This is what makes 21 powerful. 30 is the maximum level and you need to get to level 30 before you can play ranked matches. But what is so important about 20?
Runes work a lot like World of Warcraft glyphs work now. There are 4 types of runes (glyphs, marks, seals, and quintessences) and you can't mix and match. Marks go in mark slots, not in glyph slots. You need to buy the runes before you can use them but once you've learned a rune you have it forever. (Though if you want to fill all 9 of your glyphs with one you need to buy 9 of them.) You buy these runes with in game currency you win at the end of every match, though you can spend real money to increase the rate at which you accumulate them. You start out capable of buying low powered runes. (Lesser Mark of Desolation, for example, is .93 armor penetration.) At level 10 you can start the second tier of runes. (Mark of Desolation, for example, is 1.29 armor penetration.) At level 20 you can buy the third tier of runes. (Greater Mark of Desolation, for example, is 1.66 armor penetration.) There is no fourth tier.
Personally I bought a couple lower tier runes when I first started but didn't end up buying enough to fill all my slots at all let alone to have different sets for different heroes. (Stocking up on tier 1 runes seems silly in the long term since they're strictly inferior to the higher tier ones in every way except cost and when you can start using them.) When I hit level 20 I barely had enough IP (the in game currency) to fill my 20 slots with top tier runes. So I had to make some choices... Armor penetration runes are supposedly the best ones for melee characters but are practically useless for mages. Mana regeneration runes are good for characters who don't buy mana items but pretty weak for those that do and useless for those that don't use mana as a resource. I couldn't afford to buy 2 full sets. So I could either buy flexible but non-optimal runes that work for everyone (magic resistance, for example, is decent on everyone but not the best for very many people) or I could optimize one class of characters and just try to play those ones or I could break down and pay some money to get more IP.
I ended up picking a hero that was free the week I hit 20 and buying the optimal runes for that character. (Ashe, for reference, who is an archer that does physical damage and has a pathetic mana pool.) I got armor penetration for my marks, mana regeneration for my seals, cooldown reduction for my glyphs, and movement speed for my quintessences. Unfortunately most people who play in pugs don't think about team composition at all so sometimes I'd be in a game where my teammates took 2 archers on top of my Ashe. It's pretty easy to beat 3 archers (stack armor) so I'd want to switch to Annie who is a fire mage with a pet bear, Tibbers. She has no use for armor penetration but is ok with the other ones. It turned out I barely had enough IP left over to buy 6 magic penetration marks so now I'm stuck with move speed, mana regen, and cooldown reduction but I can pierce the right kind of defenses for whatever damage dealer I pick.
And then I ended up playing a tank most of the week since no one likes to play tanks but they're really important. *sigh* Guess I need to play more games to earn more IP, eh?