I've been wondering about effects like lifesteal, the mana burn from Wit's End, extra effects gained from champion abilities (like frost arrow, or Kennen's Electrical Surge), and other things like that...

How exactly do they go together? I remember from DotA there was all this bothersome nonsense about orb effects and the like, and what would and wouldn't stack.

How is it in LoL? Did the game designers decide to just let everything stack, or is there some hidden mechanical rule I have to puzzle out with my champions?

Most thing will stack, but there are some exceptions

First, anything that is listed as unique in an item description will not stack. For example, Zhonya's Ring gives +25% AP as a unique. 2 Zhonya's would not increase your AP Bonus to 50%. This also goes for active abilities iirc.

Cooldown Reduction (CDR) stacks but is hard capped at 40%

Dodge stacks, but is not additive (it is multiplicative). For example, if you have 2 itesm giving 10% dodge, they would give (100*.1)= 10% for the first item and (90*.1)=9% for the second for a total of .19% dodge (or you can look at it as each one makes your chance to be hit as 90%, so .9*.9=81% chance to be hit.....or 19% dodge).

Speed has a soft cap. I'm not sure on the numbers, but you get diminishing returns after a certain point. There is no hard cap though, in fact players have broken speeds of over 1200 before iirc (that's about 4x the movement speed of a start character, and about 3x the speed of most people endgame). Boots do not stack, but movement speed from other items (give % movement boost) do.

Aura's are a bit weird as well. You can only be affected by an items aura once from a teammate and once from yourself. So, if I buy a Aegis of the Legion Aura bonus to bothe armor and magic resist), and Larias also buys an Aegis, and we are playing with EncasedShadow, we would get the following
Encased shadow would get the aura bonus only once (he can be affected by only one Aegis aura from a teammate at a time)
Larias and I would both get the bonus twice, once from our own Aegis and once from the others Aegis.
Note that tat doesn't mean you can only be affected by a single aura, just one aura from a specific item. If I were to buy Aegis and Starks, everyone would get the aura bonus from both.

Attack speed stacks but is hard capped at 2.5 I believe

I'm sure I'm forgetting some, but those are the ones off the top of my head.

Yes. You might wanna see this Mechanics FAQ for more detailed info. The explanations are pretty updated, except maybe that for the new patch %defense penetration is applied after flat defense penetration.

You might wanna bookmark this Complete List of Guides page too. At the bottom after champion guides there are links to general guides. Might help save some time searching for things you wanna know.

I think he's reffering to 'orb effects'. Dota term ;p

Anyways, Life stealing stacks (no cap, so yes you could get 100% lifesteal)

Slowing effects are quite hard to figure out, so im giving an example:

Hypothetical Situation:
attacker has an ability with 20% slow
attacker has a Phage (25% slow)
attacker has Lizard buff (20% slow)
attacker has rylais and use an ability (35% slow)

defender has 500 movement speed

Ability slows always slow:
Phage/Lizard/Rylais'dont stack, and will take the highest slowing effect, in this case Rylai's.

That means 500 movement *80% * 65% = 260 movement speed.

FYI : i didnt take actual slow % into consideration. I honestly dont know what the slow % on Lizard is, this is just an example.

That being said: orb effects stack. You can get a slowing item, a lifesteal item and a proc item and they'll all work with eachother.

yup, ive done it.