Lets talk about the new runes.
Lifesteal/Attack Speed/Armor Penetration:
I'm honestly not sure
what will happen with this change. If people in general lean towards
Health Seals, Armor Penetration will become better compared to before. Attack
Speed also got a buff on quints, so junglers looking for clear speed are probably
going to be looking in that direction.
A quick note, if you're looking for a mix of Attack Speed and Attack Damage, you used to want Quint AD and Mark AS for more overall stats, but now it's the other way around.
With the new runes, I think we'll see a lot less Flat Armor Seals. Granted that's not saying a whole lot. It looks like both Flat and Scaling HP are strong contenders now, as well as Scaling Armor.
I've made a comparison of Scaling Health vs Scaling Armor. The choice between Flat or Scaling is not something I can optimize with math. I can tell you they break even at level 6 though, and have 3 times higher value at level 18, and that both Armor and Health have the same proportions between flat and scaling.
In general for the tanks zoning the ADC, between Scaling Armor and Scaling Health, the Scaling Armor wins out, but that is also with a lot of assumptions in its favour. Assuming 100% physical damage, with a Total HP including Shields and Lifesteal at 3000hp, you'd need ~300 pre-runes Armor for Health Seals to be best. If we consider Armor Penetration, this threshold falls to ~200.
That said, it all depends. You can try plugging in your stats in the calculator to see which proportion of physical damage you have to receive for Armor Seals to be better.
During lower levels when you haven't gotten much armor yet, hp runes hold up much better, but are still beaten by armor seals against pure/mostly physical damage champions, especially if you have sustain as your total relevant HP will be much higher than what it says on your HP bar.
Use Armor IF
You want higher sustain in lane
You're going to be taking a very high proportion of physical damage in team fights
You have really high total hp, including shields and heals/lifesteal throughout the fight
Use HP IF
You are going for/defending against all-ins during laning
You are laning against mixed or magic damage
You have low total HP
Overall, I definitely think HP seals are coming back, especially in mid-lane where sustain is somewhat rare and bursty champions are common. And of course when you lane against magic damage only champions, you can actually get runes that benefit your laning without being gimped later.
this is a juicy 50% buff. 15% CDR by level 18 is an absolute truckload
of DPS increase on most ability based champions (especially compared to
the alternative 0% on AD champs). But there are some that need to look
at this more than others. One is Ezreal, especially with the right item
build. Another more important champion to look at is Ryze. I've seen a
sad trend of not getting max CDR on Ryze even by 4th major item. With
these runes he can have many more relevant item choices. I could also
see some supports with good utility run these. Brave or ranged utility champions could make great use of them too.
They will at the very least be a viable alternative to MR and AP glyphs that we typically see, especially on choice champs.