OK, so this might seem like a pretty weird list of adjectives to describe somebody's writing style. Sure, "poetic" might seem pretty straightforward, but how can he be "subtle" and "extravagant" at the same time, and what's so special about being "metaphorical"? Well, by saying "poetic," we are trying to be straightforward; it's a basic fact that the play is written in verse, after all. (The Collard translation that we use on Shmoop puts the dialogue into prose and the Chorus's songs into verse, but in the original Greek it's all in verse – the Chorus is just more over the top.)
As for the other three ideas, they actually all sort of go together. Let's take an example from the Chorus's first song, about the Trojan War. There, they quote the prophet Calchas as wishing that "no jealousy from god / bring[s] darkness on Troy's great bridle-bit / if that is stricken first, now it goes / on campaign!" (131-133). What the heck are they talking about? The key to understanding these lines is you have to realize that the "bridle-bit" refers to Agamemnon. Huh? So, a bridle-bit is a piece of metal you put in a horse's mouth to control it. And Agamemnon is setting out to dominate, and hence control the Trojans, right? In this way, it sort of makes sense to call Agamemnon the Trojans' bridle-bit, doesn't it? But if the Trojans are the horse (pretty ironic, don't you think?), and Agamemnon is the bridle-bit, then who is the horse's rider? Well, we've already been told (for example, at lines 60-63) that Zeus is the one sending the sons of Atreus against the Trojans. So couldn't he be the rider, who is using Agamemnon as the bridle-bit to curb Troy's arrogance? That makes sense to us.
Now look at the passage again. It doesn't make any mention of "Agamemnon" or "Zeus," does it? You have to just kind of imagine them being, there, right? We'd say that's pretty subtle. But it's pretty extravagantly subtle isn't it? It isn't just implying one thing, it's implying a whole bunch of things. And the metaphors are constantly changing, one after another, from line to line, sometimes with metaphors inside metaphors. This kind of stuff mainly happens in the songs sung by the Chorus, but it also happens in the play's regular dialogue as well. For this reason, we're saying that Aeschylus's style is "Poetic," "Subtle," "Extravagant," and "Metaphorical," as well as any number of other adjectives you feel like sticking in. (We were thinking maybe "Bodacious.") This may make reading him a challenge, but we think it makes him totally freaking awesome as well.