The thing about Marathon maps is that the economy of empire development doesn't change, including war-related stuff, since all the costs are re-balanced, but the logistics of warfare change drastically.
(1) In economic matters, everyone's reaction time is essentially tripled, including yours, so long-term planning is more essential, but also more beneficial. For instance, the need for a standing army big enough to work as a deterrent is more pronounced, as is the benefit of catching an enemy unprepared.
(2) "What's the point of dragging your army halfway across the map?". I'd say fighting the enemy who matters is. As a rule you'd prefer maintaining good diplomatic relations with your immediate neighbours anyway.
(3) I haven't played that much on standard maps, but my impression is that something like a "global strategy of power projection" isn't all that feasible at standard speed on large or huge maps. Basically, the only global activity you have there is diplomacy.
The main point of attraction for me is something less tangible though: in Marathon games, an era actually feels like an era. There is a lot of time to get things done, and if you find anything to do that involves moving units around, for whichever purpose, it won't get tedious. War, long-range overseas expansion, exploration, archaeology, doing quests for city states halfway across the map, doesn't matter. In fact, I don't find the early game tedious at all since you explore at the same speed as on standard pacing, and you have the chance of exploring a large territory before the enemy expands there and prevents you.
There are two drawbacks:
(1) There can be some tediousness here and there, depending on map type and your enemies, times when "doing something that involves moving units around" isn't feasible because everything that can be done at this time has been done unless it involves attacking your neighbour. One example is the Medieval era on any map type with sizeable oceans. Marathon pacing means you have often explored everything reachable without crossing an ocean before the Medieval era even starts, so unless you're going to war, you'll have a time where you'll be doing nothing but hitting "next turn" for a while, interrupted by the occasional production change.
(2) For whichever reason, AI turns tend to take a very long time in the late game. That's the biggest issue with Marathon games. AI turns that take 30 seconds or even one minute really get on your nerves. I'm finding myself switching tasks and watching Let's Play videos of other games while waiting for the AI's turn to end - and I wish I could disable diplomacy events which have nothing but an AI leader commenting on stuff. I find this decidedly odd btw.. With computing power like a reasonably up-to-date PC it shouldn't take this long to calculate AI moves.
All in all, with any civilization I really want to get into, I'll likely always play Marathon games. Simply because, as I said, an era feels like an era there.
