1) Passenger Requests typically aren't worth the trouble credits-wise and they won't cause mission failures if ignored. Only consider them if it's enjoyable for your play style.
2) Being Allied with System Factions will net you better paying Passengers. It also helps reduce the frequency of security scans when docking, helping with completing the Illegal Passenger Missions.
3) "Sightseeing" passengers (go scan a beacon(s) and return) raise your Exploration Rank, Transporting passengers (from Station A to Station B) will raise your Trading Rank. Check the mission's recommended rank icon if you're unsure or want to double-check before accepting.
4) Transport Passenger payouts are linked to the distance of the final destination station. If you do decide to work Passengers for credits, look for a station a decent distance away from the jump-in star. I look for destinations around 100k Ls away. They're not so far that you spend a long time in Supercruise, but the payouts are still relatively good (I average about 50M/hr with VIPs, can hit close to 100M when mission rolls are excellent).
5) Named VIP Missions will hog the whole cabin, whereas "## of Passengers" Missions can stack within the same cabins if space permits (but they won't split across different class cabins). This essentially creates two outfitting strategies: First/Business Class Cabins only focusing on VIPs, and Economy Cabins only focusing on stacking as many Econ Passengers as possible. Either pay very well, so it tends to boil down to your individual play style. I prefer VIPs as it doesn't require board hopping.
6) Passenger Liners (Dolphin, Orca, Beluga) are the only ship types that can fit Luxury Cabins.
One thing I wanted to ask: see on missions about various factors that reduce 'satisfaction '.... what's that actually mean in game terms? Less money? Less reputation?
As Shiro mentioned, this is essentially a tell that the Passenger is fickle. As long as you complete without them demanding being released at the nearest station or ejecting (Mission Failures), then you'll get full rewards at mission completion.