I recently just moved to Illustrious from Pony (late to the party, I know) and I wanted to post a bunch of things I learned that don't have to do with specific settings or "HOW TO CONFIGURE ILLUSTRIOUS THE RIGHT WAY!" because none of those actually helped. It simply took a lot of messing around and figuring out the below to finally start putting out good images with Illustrious.
You don't need to change your tagging style
If you use a booru style tagging for your images then you have no need to change it. A lot of advice posts say you should change it to a specific way stating the character needs to be first, second is the general image idea, third etc. but I have not found this to be needed. Maybe there is a quality difference but I couldn't tell you. I am still using the same old kind of tagging from Pony. HOWEVER, this brings in the next point...
Less is more
Pony could work with a lot of tags very easily, at least from what I could see, but IL needs way less steering for getting an image you want. Too many tags can really muck up the image trying to bring out too much detail or having too many subjects. IL works better starting with as few tags as possible and working your way up from there. This includes negative tags. DON'T repeatedly use the same tag in a different way, ex. "motion blur, speed lines, motion lines", just try "motion lines" and see if you need more tagging from there.
Quality tags are actually needed
A lot of Pony images, and I mean A LOT, are stuffed full of quality tags at the start and I found this to not do anything at the best and be counter productive to the quality of the image at worst. Outside of a handful of Pony specific quality and negative tags, you didn't need a lot. Not none, but way less than most people use. IL actually recognizes the quality tags and having a good set of quality and negative tags will help your generations immensely. These can be done through embeds or just adding something like "masterpiece, best quality, good quality, very aesthetic, absurdres, newest, 8K, depth of field, focused subject,". I'm always hesitant about negative tags and keep the above "less is more" rule in mind but they do help a lot in IL.
Turn down the LoRA, not up
I thought Pony was very easy to set up LoRA's with. I kinda just threw them in, lowered the ones I wanted less impact for to .04, left the ones I wanted high, and literally every character LoRA I ever used was left at .08. Rarely was a LORA so strong that it affected the image so much that I had to really mess with it. IL is way, way different and this was my initial struggle with it. A change of .02 to a style LORA can have a big impact, especially among other styles. Character LoRA's can affect this too and maybe too high a setting is what is messing with your image because of the inherent style of the character. Messing with LoRA ratios is needed.
Trigger words actually matter
I feel like with Pony you could skimp on trigger words for LoRA's and be fine. Very rarely did I find keywords and trigger words made a huge difference and a lot of the time a LoRA with a high weight was good enough. In IL, keywords and trigger words make A TON of difference. Sometimes it's a little, sometimes it's the difference between the LoRA even working, but key and trigger words are NEEDED. Check your LoRA's, check your trigger words for them, make sure they're in the prompt otherwise you might think something is broken. This was a bit of a hang up for me before I realized how much it matters for IL.
Check your checkpoints
A lot of checkpoints run off the most general settings, both for Pony and IL. Sometimes you run into one that WANTS specific settings for your steps, CFG, sampling, even upscaler and quality keywords. Keep this in mind for your preferred checkpoints as it can make a real difference in quality for IL.
Image was made with Hyphoria, Dark Art Style, and Retro Sci-fi 90's Anime Style.
I am not here to offer technical help, just these small tips on getting started.