Worst in terms of broadest adverse impact? I think the biggest dent in conversion/satisfaction comes from how QM handles templates. Once a player gets to QM, imo it should not be as unlikely as it is today for them to get to 100 games and join the long tail of account longevity:
But the UX of QM, both what 5s mentioned (users often don't know why they're getting those MD games! this has been a known issue since the dawn of QM!), how QM manages templates (auto-unlock, even when it's just due to a template entering the rotation, meaning you frequently get games you don't want if you don't know what you're doing), and how QM matchmakes you (which can get brutal for noobs, the first SEAD game can suck!) inflicts imo a lot of avoidable user pain.
Imo a noob-focused UX for QM would be low-lift, high-reward for this site in terms of account retention (and, I'm saying this off of vibes without seeing the finances, but good chance this also improves monetization). It's in the site's best interest to minimize the degree to which players who've just started exploring multiplayer but don't really understand the game wind up in confusing matches they didn't ask for against players they're not prepared to face on templates they don't really enjoy playing. A training wheels experience that substitutes for what we currently have organically- players helping players at the very early stages, or players sticking around through a lot of pain through tenacity and boredom until they get the hang of the game- could go a long way. From my POV, QM template management- making it easy for players to:
1) stay on SEAD until they get the mechanics figured out (no auto-unlock that confuses you by throwing you into SE1W or the esoteric templates, when you're at like a 30% wr on SEAD!)
2) learn the game, or at least learn SEAD, with a lot more handholding than they get on the Tutorial
could go a long way. The site's directionally
moving toward (2) - like with the highlighted minimum armies for an attack to succeed - so (1) is what I'll use this soapbox to whinge about.