1) Always advance your primary attack ability score.
Definitely. Any other choice is terrible over the career of your pc you'll do significantly less well offensively if you fail to push your primary at every chance.
2) Advance you second ability score to improve secondary bonuses and to meet feat prerequisites.
depending on your build and class there's a lot of various ways to go here but obviously most pc's have at least one stat that is pretty important to their build beyond the initial. This will almost invariably pump one of your NAD's as a secondary effect (other than dex for wand wizards I can't think of another case where this isn't true).
3) Improving defenses is a tertiary consideration for ability score increases. Rely on magic items and feats to improve your defenses, plus situational power boosts from yourself and your allies.
Hmmm, you want to avoid damage as much as possible, I don;t think you can solely rely on feats and magic items or you will lag behind and get hit a lot. Will and Fort are targets only slightly more than 25% of the time so it's not a fatal flaw if some encounters make this a liability, especially if the party has pc's with high fort or will which they likely will not all the pc's will be totally vulnerable to being over matched by any particular group of monsters.
3 levels over the average party is the maximum suggested by the books. Higher than that, and you most likely (very likely) will kill your players. TBH, it's fairly easy to do so at +2 already
4 levels above party level is the max suggested not 3, (pg 56 dmg). Keep on the Shadowfell has two encounters at level 6, one is guaranteed to be met by 1st level pc's if this is your first campaign. So while 4 is suggested 5 is also quite possible. N+2 is not going to often TPK your party. Very infrequently in fact. N+3 and N+4 are where the rubber meets the road in 4e. The most exciting, challenging, and rewarding encounters will leave you with the "whew, that was close feeling" which enhances most players sense of accomplishment.
In my experience DMing and playing, being one dimensional should not be the bad word that it is often made out as. Each core class has defenses that they will tend to be good at. The paladin has a +1 boost to all three, but trying to 'keep up' with all three will waste resources and leave you with fewer options.
trying to keep up with all three will just make you particularly vulnerable on average. Many monsters have really high attack values. Better to be hit by a 4 or 5 vs one defense than a 7 vs all 3.
At the end of the day orcus dropping one level 30 pc to 0hp's in round one won't be ending the battle. The cleric could cast a low level utility and put him back up to 50% without spending a surge. He can drink a potion as a minor and be around 75%. Meanwhile the rest of the party is going to work on orcus. The power in question is a recharge 6, most level 30 parties can shrug this off at least 3-4 times in a battle. Assuming it hits 65% of the time this means it's going to need to recharge 4-5 times at least before it becomes encounter defining. 20+ rounds is a long time for orcus to stand up to 5 attackers with level 30 powers.
Does the 29th level warlord power defy death block half this damage on a miss and all of it on a hit? that really puts a hurting on a recharge 6 power.
I don't think the Achilles defense suggested earlier is a good idea because the party could get hosed and tpk'd by a relatively weaker encounter that had a homogeneous group of monsters who target the "heel" defense.
