Please don't flame me, but one thing that EverQuest taught me was that hit points rule. I observed there that the leading cause of character death was insufficient hit points, or looking at it another way, hit points = stupidity resistance. I made millions of GP in the bazaar selling HP gear, and the best way to twink a new character was HP gear. Now I realize that EverQuest is a different game from 4E, but 4E is definitely informed by MMORPGs, including EverQuest.
There is no doubt that Hit Points are central to 4E's mechanics. Class balance depends in large part on differences in hitpoints and healing surges. Notice that Twin Weapon Fighting Style rangers get Toughness as a bonus feat, and not Two Weapon Fighting. Look at all the print devoted to temporary hitpoints.
One more CON bonus means one more healing surge, means an additional quarter of my base HP's worth of fightability per day. I'll take at least one, and if I had a larger point buy, I'd at least look at squeezing another CON bonus out of it.
Yes, I did learn from EverQuest that Damage Per Second (DPS) rules, and it does in 4E as well. We do what we can to boost it, mostly pluses to attack rolls, and to a lesser extent, pluses to damage rolls, but that only goes so far.
Well, another lesson learned from EQ is kiting, that is damaging melee focused creatures at a distance, so that HP and AC matter less. But as anyone who has kited has learned to their regret, sometimes things just go wrong, and HP is your insurance against things going wrong. The more HP you have, the bigger blunder you can withstand and recover from.
I think this applies to 4E as well.
My gut tells me never to play a 4E character with less than 12 CON, and I think my gut is right.
Assuming you used all your healing surges in a day, here are the total HP available for 10, 12 and 14 CON starting PHB character classes (Base HP + Surge Value * Surges Per Day):
Cleric: 57/72/ 80
Fighter: 79/87/106
Paladin: 85/93/113
Ranger: 52/66/ 74, or 63/78/87 Twin Blade
Rogue: 52/66/ 74
Warlock: 52/66/ 74, but CON tends higher
Warlord: 57/72/ 80
Wizard: 50/57/ 72
For all classes, a CON of 12 or higher is much better. Knowing that you could step on a booby trap, get shot by a sniper, or have a poisonous gas cloud dumped on you at a moment's notice, who wouldn't want to have the most insurance they could afford?
If it was you, and not just your character, you'd choose more HP every chance you got.
That being said, the Array/Point Buy system insures that there trade offs must be made. Attack is mostly much better than defense, and certainly more fun, but I consider a CON score of 12 to be the minimum playable, and am always tempted by the prospect of a higher CON.
I have not yet played a back rank character, like a Warlock, Wizard or Archer Style Ranger, but I suspect that regardless, damage is the most common danger any character faces, and a good referee will stress the back rank whenever possible.
Make Mine Hit Points!
Smeelbo