My top reasons to fly UA: (I'm Platinum).
1. Price (which IMO should be everyone's #1. FF plans ought make the difference between airlines when price is the same.)
2. Bonus miles as an elite member
3. WN & DL FF programs suck (WN is $$ based and DL has high award levels). BTW, I used to include UA's FF program in the "suck" category owing to *blocking.
4. Fee waivers (SDC, award change/redeposit, baggage)
5. Network allows multiple connections to maximize flying experience and earnings
6. Front-end (and back office) staff
7. Great hubs (esp. EWR, CLE, ORD)
8. Loyalty to the memory of CO and remaining staff from the good-old-days
9. Good intra-Europe earning opportunities; don't have to fly AF
10. Partnership (for the moment) with US
11. Ch 9.
12. Friendly web interface
13. Can generally avoid JFK, DFW & ATL
14. Good VDB history
Things which have absolutely ZERO impact in airline selection:
1. Lounges
2. E+
3. Upgrades
4. Global entry
5. F/C product
6. Frequency
7. New fleet
8. Payoffs for IRROPS
9. Jeff