There's a whole bunch of things I would fix, but three stick out from a user-experience perspective:

• Better wayfinding signage. IIRC, Randy said they would soon launch a major improvement project in this area. Reintroduce the "X Line Train to Y via Z" signs on the platform. Actually roll out signs at stations that show what line/platform goes to DCA + IAD, not just IAD, as they have recently done.
• People may seem this as a nuisance, but figure out a way to get the PA system in stations to call out the destination/line of approaching trains. The PIDS boards are often hard to read, especially at a distance. Other cities have such a system, but that said, I feel like WMATA makes more overhead PA announcements than most systems...likely why they got rid of that European-train station "ding-dong" that used to play before announcements.
• More Spanish signs and announcements. It wouldn't necessarily be hard to program the 7Ks to make important announcements, like "Transfer is Available to XYZ," in Spanish. I've frequently read that the system is somewhat hard to navigate for Spanish speakers who know little English.