All of what Sapyx said. Additional differences you might find relevant...
All factions have independent alignment.
The number of native factions per system is considerably lower. There is one system with three native factions, a few with two, and most only have one. This makes it harder to use factions as "padding" because they're likely to retreat if you do. The expansion patterns this causes mean that for almost all factions the lower-priority states are very short-lived, as factions end up in Expansion, Retreat, War or Election very often.
Populated system density is low - though overall system density is of course much higher. Colonia has 8 inhabited systems within 15 LY (plus a detention centre) ... Sol has 32. This causes some of the outlying systems to have relatively poor mission choice (though there were big improvements made in the 2.3 and 2.4 releases). As a fallback to this, four systems - Colonia, Ratraii, Ogmar and Tenjin - are recipients for mid-range missions. Pretty much wherever you are you're likely to see some missions to those four. All systems have populations under 500k, so small actions have large BGS effects.
Colonia and Ratraii have BGS protection - no expansions into those systems, no conflicts in those systems between factions already there.
Other than the locked Colonia and Ratraii there are only a few systems not yet under PMF control
- Trakath (taken by Jaques early on, would need a Retreat to open up space or an Invasion to bypass that). There is one player faction present but they're not expansionist.
- Tir (had lots of NPC factions expand into it early on). One player faction has got in, but is not yet going for control.
- Einheriar (position made it hard to expand into). An expansionist faction has recently reached it and will probably obtain control soon.
- Farwell (originally PMF-controlled, they lost control to an NPC faction). They appear to be trying to take it back at the moment.
- Pekoe (position made it hard to expand into). Two PMFs have now expanded into this system and one of them is likely to eventually control it.
We are expecting some additional "outposts" to appear soon as a result of the Colonia Exodus CG - it's not clear whether this represents more outpost-class stations in existing systems, or new systems.
On ethos: a lot of the NPC factions have government-ethos combinations copied from the PMFs, so they won't necessarily be what you expect either.
Full current information and history for the past year is available from the Colonia Census -
https://cdb.sotl.org.uk/