From Gold to a most reluctant 1K, strictly due to UA's monopoly on my most common route. Took DL whenever I could -- likely cost me GS, but I plan to continue to avoid UA as much as possible.

In response to the request for reasons, they are manifold:
1 - Far and away the biggest reason is lack of reliability. There were a couple egregious examples but the number of trips with non-trivial delays over the past year was staggering. There is also a troubling pattern of flights arriving late and then departing even later, with zero sense of urgency on behalf of the ground staff.
2 - Piss poor handling of irrops. Multiple instances this year of being rebooked to unacceptable alternatives, often not in my ticketed class of service. Most travel is OPM so refunding the difference does little for me.
3 - Group 1 insanity -- it's just too big. My spend was circa $35K on UA last year; the GS flanking maneouver while I still have to show up well before boarding time if I don't want to play the game of swimming upstream to get my bag from further back in the plane because magically the bins have filled up is kind of a slap in the face.
4 - Onboard services are egregious for the cost of F. More important than anything else, I'm tired of the dice throw for a plane with operational power/wifi - I travel for work, so I need to... you know... work.
5 - Constant moving of the goal posts on frequent flier perks (feels like there are material changes to the program every 9-12 months). Pick the program and stick with it. Not that others do better, but I'll just give up the game altogether at some point.
6 - Continued cuts to SJC service not appreciated

Didn't make the list because it's kind of overarching, but I just don't trust United; multiple breaches of trust and lack of fulfillment of promises (including basic implied promises of the brand) has left me feeling antagonized every time I fly.