I watched the first episode of this last night and had a few thoughts afterwards.
Firstly, I thought this was meant to be a reboot? I thought maybe they'd take Jack and Chloe, put them in London and basically start from scratch. The only back story you need is that she's a hacker and he's a badass.
Instead of this, they decided to continue on directly from season 8, which happened 10 years ago, and refer constantly to shit Jack did in the past that everyone was angry about. Then they introduce some other characters that have back story with Jack who you would only know if you'd watched seasons 6-9 or somewhere around then. If you're trying to get new people onboard, it seems like maybe you shouldn't make them feel like they've missed out on a shitload of things?
I've watched season 1 and recently season 7-8, there really isn't much that follows on from season 8 apart from Jack being branded a fugitive (just dumb governments trying to threw the blame somewhere and cover their own asses). There is less follow-on in season 9 from the previous season than the follow-on the other seasons get from previous seasons.
The events in season 8 happened 4 years ago from the timeline in season 9 not 10 years, back than someone close to Jack died and the Russians were involved, so Jack did what he does best and served some justice on those involved. The American and Russian government didn't like that though, even though Jack was only cleaning up their mess, but...well, governments suck. Now you know why Batman wears a mask, so he doesn't have to deal with that crap.
The story behind Jack and the current American president and his daughter seem fresh, I believe he met them 3 years which wasn't covered in any of the 8 seasons - well I know for a fact that they weren't included in season 7 and 8. I suppose during the season the story behind those characters will be covered, including Agent Morgan and the story behind why she didn't realise her husband was leaking American intel to the Chinese.
Just found out, the story with Jack, Heller and Audrey is covered in season 4, so there is backstory to the current events in season 9. My bad =P.