This warlord power we are talking about (that gives +4 to all defs is 29 daily so we don't have it now!).
Actually it isn't. The PHB wrongly attributes the power to the warlord. The power used in the example is 'Hallowed Ground', a cleric 16 utility power that also increases the party's attacks and saving throws by 2.
If it had been the power you mentioned, all defenses would have been 2 higher than I wrote in my previous post. But, anyway, I think you're missing the point:
This power is just an example for all the powers that will be available to a party of that level. Your math is not taking the existence of _any_ of these powers into account. In other words:
Your math only works for partys without a resource they're expected to have.
Secondly this is solo artilery so ithings get more complicated.
Actually, I think it's easier, there's fewer variables to take into account
Normal monsters team will have first line (soldiers, brutes) and second line (arilery), but that is not important. Let's crunch your example.
Well, you've been the one asking for a level 25 artillery monster. As I mentioned, there aren't any others in the MM
The AC is fine by RAW so we don't need to calcute this. Even if some one need to do so, this Naga is Artilery so it's to AC hit chance is lower than other monsters (it should be +30 on average), Nagas is +29.
Well, imho, real example >>> hypothetical example.
I agree in this fight mark will do much. But in fights were are more monsters hitting NADs and first and second line this will be not much factor, couse defender will can't mark artilery so easily and often he will not even try to do so.
I disagree. Experienced players who are tactically savy (and they will be, otherwise they wouldn't have reached level 25 in the first place!) will always use their abilities to best effect. If the fighter isn't close-up to the naga, he'll be fighting something that is an even greater threat (which in this particular example is unlikely, unless the encounter is of a significantly higher level: about level 28-29).
Even if this will be a fight at 29 level and warlord will use his power (Stand Invincible) that gives +4 to all defenses, you must still rember that this is a daily power! And can apply to only one fight, but it will be porobably saved for the final battle. If you don't have warlord I don saw any other power whose looks like this on other PHB leaders and paladin class. So it is very, vetry situational.
See above.
If the party has a leader it's extremely likely they will have a power similar to the one mentioned above. If you haven't found any, you weren't looking (there are, btw. also powers that will cause additional attack penalties which serves the same effect as increasing the party's defenses AND will stack).
You're right though, that, generally, powers that have effects that last for the entire encounter will be daily powers. But:
You have a party of five characters. At level 25 each of them will have 4 daily powers and 6 utility powers which may or may not be daily powers.
Now, 4E doesn't mention how many encounters you'll typically have in a single day. In 3E it would have been 4 encounters, i.e. on average each character would have one daily and one or two utility powers left.
If they had already blown all of their daily powers it's unlikely they would have continued and thus wouldn't even have this encounter in the first place!
Finally, you're simply dismissing encounter powers. Each character will also have (at least) four encounter powers. Even if each power's effect only has a duration until end of next turn or (save ends), that's still four rounds for which your assumptions won't be correct.
Let's crunch this numbers.
[...]
Let's take form simplifiaction that other will have the same stats (they will vary in with DEF but that is not much important).
BEEP! I disagree. It's extremely important to remember that not everyone in the party will have the same weaknesses. If the party is balanced, on average, one or two characters will have REF or WILL as their highest defense.
All attack will be at -2 becous of fighter mark.
So FOR will be hitted on 8. REF on 5 and Will on 3.
Much better, but not as impresing as it should be.
Says who?! You're forgetting the distinct advantage the party has in the number of actions over the naga. The naga's attacks have to more accurate than the party's to compensate for that! Otherwise it wouldn't do enough damage to be a threat.
This is 5 players party (all melee fighting). So there will be 5 OA when Naga will make his minor attacks. But this is pure fantasy.
... which is why I didn't say, there would be 5 OAs... and, as I already mentioned: shifting to avoid AOs isn't free: it will cost the naga one of it's minor actions which it could have used for an attack. A monster that isn't attacking isn't a threat, regardless how high its to-hit chance is!
If she uses his close blast 5 attack, even fighter with his strong FOR will be hitted on 6 on die. His highest DEF!
Well, I think, hitting on 8 is probably closer to the truth, but anyway: remember, this requires a standard action and can only be used once every three rounds on average (it's recharge 5,6).
You make a very specific example here.
[...]
+32 vs. 41AC - 9 on die to hit (55% chance; +1 player adventage).
+30 vs. 41AC - 11 on die to hit (45% chance; -1 player disadventage)
+28 vs. 41AC - 13 on die to hit (35% chance; -3 player disadventage)
So we will take average to hit that is +28. 35% chance to land a hit. Becous CA is situational! On 1 level you chance to hit is at +1 adventage (+3 with Combat Aventage!).
And again, you're not taking into account any powers that might increase the fighter's attacks. As mentioned above, even the level 16 utility would add 2 to all attacks.
Anyway, actually, this is a closely related but different discussion. THIS thread is about the PHB defense feats, right?
The dmg given, taken is not important here. The fact how easily Naga can hit enemies is very, very broken and this is my statement from the beginning.

You cannot look at the attack bonus in isolation. Attack and damage are always related. It's why the avenger's damage output is similar to the other strikers without requiring situational bonus damage. If attack chance and damage weren't related the avenger wouldn't be a striker
The naga's damage is actually quite low compared to the damage from encounter powers available to a level 25 party. Especially the damage from its minor actions is closer to the party's at-will damage. So, since the naga has fewer actions, it _needs_ greater accuracy!
This isn't brokenness, it's balance!
Let's look at another artilery example. 24 lvl artillery - Greater Flameskull.
[...]
This attack makes 2d8+10 dmg (19 dmg on average) and makes target dazed until start of skull next turn, so nobody can help you with this effect.
Well, I'd rather not, at least not in this post. I'd also recommend to take advancing monsters out of the equation. I'd rather look into monsters as written, because they're closer to the designer's intent.
Yes, daze is a nasty effect. But it only affects a single character and the party will have similar effects available to them. So, unless the party's encountering more greater flameskulls than there are characters in the party, I wouldn't be overly worried.
Flameskulls also have a very distinctive weak point: they're undead. You'll probably dismiss this, because you haven't modeled this in your simplified math (and probably can't), but it's an important aspect of this particular monster. It's an easily recognizable weakness that _will_ be exploited by the party.
But this are easy monsters. Look at those who dominated/stunn/weaken and have powers like Bodak, Wraith, Orcus etc.
Yes, bodaks and wraiths are particularly nasty monsters (although they share the weakness mentioned above: they're undead).
Now the question is:
Are these monsters nasty because of broken math or because of the way these monsters have been designed?
The needlefang drake swarm is an often cited example for a (very) difficult monster. Does this have anything to do with the math? Nope.
In other words: Not all monsters are created equal. There'll always be some that are more difficult than others, so you'll have to be careful when designing encounters.
Defense feats are must-have or must-have for people who know what are consequences of not taking them.
It may be obvious by now, but for the record: I'm unconvinced
Probably they don't know how much impact this was having in long term. Maybe it looks weak but when you have to face one fight. If character will be hitted one time, this will not have any effects. But when hitted 100 times, it will means that 10 of this attack will be a miss, 90 still a hit.
Well, opinions vary on this. Many players think that the best defense is a good offense. Looking back at my experience with 3E, I'd have to agree. Is it also true for 4E, epic or no? Only time will tell.