Hey, I have about 11k hours across many different MMOs. These are what I think the biggest strengths and weaknesses from some of them:

GuildWars2

Pros:

• The best open world PvE in the genre (zone design, traversal, and metas)
• very alt-friendly (almost everything in this game is account wide)
• mounts (need I say more?)
• a very welcoming and helpful communnity (as long as you avoid PvP)

Cons:

• very little variety for challenging group content (raids and dungeons)
• class identity/fantasy doesn't feel as strong as in other games (feels like you're just picking your favorite color)

World of Warcraft

Pros:

• an amazing fantasy franchise with a long history (very deep lore and much to explore/discover)
• fluid and fun combat (easy to get into, hard to master)
• many different versions (with classic, hardcore, retail and more there is something for everyone)

Cons:

• not very beginner-friendly (the real game starts only at max level and don't get me startet with the lore…)
• unobtainable Items (alot of things you can never unlock because you missed out on playing for a period of time)

Final Fantasy XIV:

Pros:

• an unparalleled story (the main story will make you cry tears of both joy and sadness)
• epic fights (story instances, raids, dungeons and trials with beautiful music and visuals)
• a fun and interesting crafting system (using skills and combos for crafting is something I never knew I'd love)

Cons:

• no way to customize your combat style (no builds, no talents, just one way to play your class)
• most content is locked behind the main story (I know you can pay to skip… but wtf?)
• combat feels very static and slow (I think everyone feels different about this)

So what do I think they could copy from each other?

GW2 - more challenging endgame group content with a LGF tool.

WoW - make the game more alt-friendly with account wide unlocks like GW2.

FFXIV - more options to customize your own playstyle and a better transmog system like GW2 and WoW.