us works the same, same for upgrade costs
2x speed for settler/worker is hot and not so hot; on deity I find it completely very not so hot from the human pov. Same goes for cultural borders; many many turns to last in there. Actually I find culture a royal pain on marathon.
Nationalism without spiritual is perfectly way worse on marathon imho; that's why I tend to give spiritual #1 in a traits chart, while people playing on normal seem way less impressed with it. The idea is that your globe city which is able to grow from pop 5 to 6 in 1 turn, here grows in 3 turns. Whipping units also loses some value, though I find whipping units still king.
barbarians; at least on deity... better have some good spots for fog busting. First times I was playing on deity I was wondering if the computer makes fun of me. Also pray that your neighbour doesn't build gw, or you'll look like grand central
diplomacy - I'd say it's sensibly harder; with 3x more turns, you'll have 3x more opportunities for someone coming to ask things; while the settings for turns inbetween asking are the same. So a long war between 2 ais will surely give you the opportunity to decline joining the war 2-3 times. This together with culture I find the biggest problem on marathon; though deity barbarians are nothing to shun at either.
Also, you'll be checked for war declaration x3 times
golden age - worse; 2x length for 2x unit production cost is par with normal; same for civic swapping. Buildings - worse then normal. Gold/research - worse then normal, since techs/upgrades costs 3x and your ga lasts x2
but thank God, all this is compensated by the fact that units actually move at a decent speed compared to the time needed to build them.
other marathon bonuses:
1. mines; given so many turns, and since it's a random chance/turn to pop. a resource from a mine, it's wise to build forges. The mine resources will most probably pop sooner or later
2. wfyabta - there's a 5% chance of forgetting a trade each turn. With 3x more turns... I seldom find myself bothered by wfyabta.
3. random events. Start game, they can screw you up much easier on marathon; on long term, I'd say it's a bonus, since overall, if we're to be honest, they're more on the positive side.
4. spikes/rushes - obviously it's easier to spike your power and declare. Same probably for axe rush, though it was a while since I did my last one. Though ok, the rush is abit limited by the fact you can whip units sensibly less efficient.
5. healing - what's a mash?

Though you'd still better get a mash out.
hope it helps