Speaking from experience:
a) Factors which would make me leave:
1. Unhealthy work life balance (and by that I mean I need/want REAL BALANCE, i.e. regular 40-45 hour weeks with the odd peak once in every blue moon. anything else is unsustainable)
2. Having to stay away from home too much. I want to spend most nights in my own bed, seeing my wife and kids in the evenings. Companies which ignore this point usually only end up with young graduates and 'odd folk' working for them
3. politics, manipulative colleagues, bosses who talk about things behind the scenes, backstabbing during appraisals, etc
4. uncertainty/risk combined with pressure, e.g. an 'up or out' culture. Many folk want a steady job, they have mortgages and commitments dammit
<b>Very important note:</b> You will note that pay does not factor into the above list. This is not to say that it is not important, however. The reason it seems to be at the top of so many people's lists is because points 1 - 4 are not being truly satisfied (despite what HR might say).
b) Factors that would make me stay:
1. If factors A1 - A4 aren't present, then the only thing that would make me stay is a very, very, very substantial pay package. Otherwise it's simply not worth it.
2. A 'home working' policy so I can actually be with my family
3. Minimal travel or staying away from home
4. People I actually like to work with
5. A stable team (i.e. without being disrespectful as I very much like our continental colleagues, I'd prefer not to be thrown on a team with a different random cross section of the EU every 2 months. my foreign language skills aren't up to much either, but pigeon english drives me nuts, plus these people have different interests and tend to 'come and go' making it difficult to form any kind of productive relationship)
6. a nice chunky guaranteed bonus, to the point where I feel like I'm really paying off the mortgage in bounds and leaps and can see myself getting out of the rat race within a few years
As it happens, I found the working conditions at these big companies to be so bad that I decided to set up my own firm, which is successful and has been running for over 10 years now. These big companies take too much and give too little in return.