Let's summarize what we know so far about 1.9

NOTE: Take all current lap times with a grain of salt. There have been patches, bop changes etc. while doing the laps. So things will get updated as we go along. Kunos usually also still finds issues once the game is out in the open and more people get their hands on it. So there will likely be hotfixes before lap times stabilize.

Temperature

There are some changes to the tire model that result slightly different temperature behavior

Temperature does not sink into and out of core that quickly anymore

1. That means that tires take longer to heat up, we've found 3-4 laps before the tire has built temperature and achieved the target pressures (whatever they might be). On shorter tracks this can take longer.
2. Tires cool out slower, both at the start of the race, but also after a spin or mistake, it will take a bit longer for the tire to come back to normal temperatures. That means that if you get it really warm after a mistake, you must take care for a bit longer to not exaggerate the issue

Surface temperature is more dynamic

1. Surface Temperatures will decrease more on the straights leaving you with often cold surface as you approach a corner after medium and especially long straights
2. Surface Temperatures will increase faster during cornering and wrong camber or toe setting might lead to extreme heat

Temperature spread from inside to outside on the tire is larger

1. Of course this depends on your camber and toe settings, but due to the changes running negative toe will now drag the inside of the tire over the tarmac in all situations but turning, resulting in the inside never cooling, while the outside of the tire cools a lot, leading to large temperature and likely wear differences from inside to outside. If that is an issue is yet to be seen. I personally found the rear to be rather snappy on entry with negative toe resulting in inconsistent driving
2. Positive toe can help also utilize the rest of the tire more during all phases and especially under braking more tire might be in contact with the road decreasing the braking distance (yet to be tested and confirmed)
3. An example on the barcelona straight coming out of the last corner. The front left tire will leave the corner with something like 96°C on the inside, which will cool down to 78°C on the straight. The outside of the tire however leaves the corner with 100°C but cools down more to 65°C over the straight (taken from Aris stream, camber and toe will affect this massively)

Impact of alignment

Toe and camber will affect the spread of the surface temperatures and how much the wear differs from inside to outside. We couldn't find extreme issues with using extreme values, but we also didnt find free lap time as before. Slight negative toe might still work as it keeps the spread small enough to not have negative side effects. Imagine it like this:
Negative toe with a car going straight will lead to the tire being "pushed" towards the outside edge on the rim, pushing the outside of the tire further away and due to the negative camber up in the air and out of contact with the road. Under Braking the forces on the tire act in the opposite direction of travel, thus exacerbating this effect. So you are braking on the inside flank of the tire (not actually, but exaggerating the thought) and only have little contact patch, extending the braking zone and heating up the inside much more than the outside. With positive toe then the tire would be pushed towards the inside of the rim "pulling" the outside of the rubber closer to the ground and especially under braking the outside of the tire will be utilized more, giving more contact patch, potentially shorter braking and more equal temp spread. The same logic will be true for camber even though near max still seems the way so far.

A suggestion by Aris is to create rotation now by using less rear camber, less pressure and positive toe to overdrive the outside of the rear tire and force it to slip. We can confirm this effect yet we are unsure if its ultimately faster. 

Unfortunately there are indications that negative toe will still lead to outright faster laptimes at least in hotlap mode due to the temperature and tire reset. Practice and races where temperatures play a bigger role might yield other results. So we'll monitor how this develops.

Here's a video with road tires, so it doesn't quite translate, but it can give you a nice idea i guess.

Pressures

1. The pressure window is now roughly between 26.0 and 27.0 - it's not as punishing outside of it but the tire will feel different nonetheless at different pressures
2. higher pressure will be needed in fast high load corners on high downforce tracks. It will develop marginally less temperature and feel "sharper" or more direct, more responsive, communicating a clearer grip peak
3. lower pressures will help in slower corners in turn and with lighter cars, quali fuel loads (potentially), however they flex more and generate marginally more temperature, while the tire communicates less clearly how much grip there is, it will respond less directly and might feel a bit vague. I guess those differences will be more pronounced on DD wheels - so why not use one of the affiliate links on the right? ;)

Suspension

Previously there was an issue where the suspension travel was unlimited. While the bumpstop could be more compressed and would become infinitely stiff, there was no final end to travel and that allowed for weird suspension setups, leaving the springs on the softest settings in almost any scenario.not endless travel anymore (with infinitely stiff bump stops). 
Now however, there is a definitive end to the suspension travel and any excess energy will go into the chassis - which is where you do not want the energy to go. So you need to use dampers, springs and bump stops to prevent this from happening, often leading to stiffer springs used. With that comes a generally more stable aero platform which makes the cars more pleasing to drive in general, less sensitive into fast corners and less understeery for aerodynamic reasons on exits.

Qualifying

It qualifying you now might need a lap longer to heat up the tires and produce the grip. Also the initial wear you need to pass in order for the tire to produce max grip has been altered slightly. Both temps and "grooving in" of the tire take longer. Lap 1 is not going to be your fastest lap.

Race

Temperatures rise for a bit longer into the race so its likely you'll start with even lower pressures and have to wait for a long time for the tire to reach their window. Also it seems like mistakes will lead temperature in the tire being raised for longer periods of time and come at a bigger cost. Yet, there's lots to test and reveal still. It's still early.

Here you can find Aris prepared Document for the 1.9 physics changes.