If I MUST fly coach (and I try and avoid it on flights >2 hours, i.e., by maintaining elite status and getting upgrades), then I go for:

1. aisle seat -- mostly for easy bathroom access without having to bother others, as I have a medical condition that sends me there at least hourly (and that's if I DON'T drink wine or coffee!)

2. a seat with room to put stuff under the seat in front of me -- nothing worse than having all your stuff stuck in the overhead bin during a prolonged bout of turbulence when you're not allowed to get up and get it
[beware that some aisle seats have more limited under-seat capacity than middle or window seats, one definite disadvantage of my needing an aisle seat]

3. bulkhead avoidance -- too much risk of being near babies the airlines tend to seat up there

4. not too far back in the plane -- the legroom is often less and the plane is often noisier in the back

5. definitely not right next to the toilets (despite my interest in going to them ) on long flights -- as noted above people (often drunk people) hang out next to them and often the aroma you get with the bathrooms doors repeatedly opening and closing is nauseating

6. another consideration is importance of overhead bin access: if you bring on a full roll-aboard plus a "personal item" and you are seated where you are one of the last to board (i.e., front of plane without elite access), then by the time you get on you may find no room to put your roll-aboard and it will have to be checked.