I can answer this two ways:

I used to use CO as an alternate carrier to UA, and even made it to the lowest level of elite a few times, but when int'l upgrades became impossible for non-elites, and almost impossible for low-level elites, I did leave, and now fly UA 98% of the time. I took my first US flight in years on January 2, 2003, and I expect that US will get me to the few places that I can't get to on UA.

From another perspective, although this really answers the converse of the question asked, Economy Plus is what keeps me on UA; that is, as long as they have E+ exclusively I will stay with them regardless of other considerations. If they took it out I would probably switch to AA.

Upgrades
Elite Checkin
Priority seating (or in the case of some airlines even if you book 21 days out there are no seats to assign unless you are elite.) Take that away and that is a quick exit. If I can't have a seat assignment I may as well be on Southwest, at least I am not automatically doomed to a center seat, and they actually provide service in flight on short trips.

Now these are not costly items for the airlines, the elite checkin doesn't add another employee it just takes one away from the regular line. Upgrades, yeah right lots of people buying first class seats these days. Seating assignments, give me a break where is the cost in that?

and for United, Economy Plus seats go, I go.

What I would like to see is easier redemption of awards. Southwest is the only one that actually does this, and they always have a lot of award seats available - if there is a seat on the plane you can get it as an award ticket, try that on another airline. So even though Southwest doesn't offer the things that would drive me away from the other guys, they offer the advantage of easy to use awards.

For those who are not anal retentive, those of us who are are constantly figuring and re-figuring how much time we have before take off: "Let's see, 10 minutes at check-in, 15 at security, 5 on the train to the terminal, 3 for restroom, that gives me 15 minutes in the lounge before I need to walk to the gate."
Therefore, my order is:
1) elite check-in, (though not always the best because if they only have one agent it goes slower)elite security lines
2) 100% mileage bonuses
3) Upgrades (would be higher on the list if I got more of them. This will change when I hit 1K next month http://www.flyertalk.com/forum/biggrin.gif)

Now that I look at the thread, most of us would be disqualified because it clearly states "...took this ONE perk away..."

http://www.flyertalk.com/forum/tongue.gif

Stopped giving me a chance to redeem those miles for a seat.

Everything else is just gravy. You folks are forgetting about the meat and fixating on the garnish.

1) wide availability of awards, especially intl. C
2) wide availability of intl. upgrades
3) elite check-in
4) select seating

1. Lounge
2. Miles that can be used to fly (hey don't forget those)

Howdy,

No more free seats in F for my family on vacation - and I'll leave (using miles of course).

Now - if a 'perk' isn't an award seat.

Then it has to be upgrades.

Keep the faith,

Pakse

Sorry you feel that they have nothing else to offer. I agree that the upgrade policy is the best there is, at least at the Silver (or equivalent) level. But I don't think anyone comes close to the ease and speed (and availability of seats) of the online award travel booking process. Why can NW allow you to book today, E-ticket you, and allow you to travel the next day while Delta charges that ridiculous $75 'Expedite' fee?

Would leave CO for AA if:

CO eliminated unlimited upgrades OR pre-boarding of status flyers.

1) Elite check-in lines.
2) Priority upgrades based on top tier. UA enables 1Ks to upgrade 28 hours before the next level (1Ps) and 76 hours before 2Ps.
3) No award travel blackout dates.
4) Dedicated CS and reservation phone numbers.