Someone mind explaining a tad bit better as to what of those three situation that the Dread Necromancer can't do?
1. A DN and its minions can easily get through traps(minions being disposable) and kill a dragon.
2.DNs are armies in their own right past level 8
3.The social situations is where I see they would have problems with, but they're a CHA class with powerful fear abilities and bluff/intimidate. Intimidate is an awesome counterpart to diplomacy.
The dragon thing is just one example. But DNs have few social skills (Bluff on a CHA class is nice, and Intimidate has uses, but neither are good if you want to be long term allies with whoever you're dealing with). Stealth is an issue, as they lack Move Silently and while they do have Hide, their minions are usually completely incapable of stealth (and the better minions are usually too big to just throw in a portable hole). They're also very dependent on finding the right stuff to raise... if the DM isn't throwing hydras and giants at you, that can be an issue (I know it has been for me!). Most undead you can make until you get Create Undead is just a speed bump anyway, with lots of HP but generally poor attack power. There are exceptions... but only if the DM gives you the right stuff to fight. Spend a while fighting humanoids or really anything with class levels and the zombies/skeletons you could create are just plain sub par.
Now, you've still got a few solid spells to work with (Evard's Tentacles, for example, is quite nice). But you generally don't have the really powerful ones (Planar Binding you have, but not Magic Circle or Dimensional Anchor, so it takes a lot of work to actually break that), nor have you got seriously potent undead unless the DM handed it to you.
End result... you're solid in a lot of situations, with enough abilities that in most any situation you've got at least something useful. But you're not really game breaking unless you really work for it. And that's basically what Tier 3 is. You can be useful in any situation, but you've got weaknesses too. You're unlikely to just break the game, nor are you likely to be completely useless.
@Tenebrus: Yeah, basically. The Fighter is good at combat. That's cool, but that's it (generally). And at higher levels, they start to become quite poor even at that unless you optimize them quite significantly. The Ranger, however, is going to be useful in more than just combat, and they can be quite good at combat too.
JaronK