There will be all kinds of buffs?or buff lines.
As far as what is needed to be effective, you will not need even half of them at any given time, being that they will all have specific needs.
Bonus to Ice defense is not needed when fighting fire creatures or wind creatures or animals, but when fighting ice creatures could be invaluable.
I know this does not help much, but there are several schools of thought in the dev team as to who gets what when.
Personally, I think we will see many general mid length buffs (lasting 20 minutes or so) but most of the really nice buffs will last only a minute or two.
Will buffs be needed to go against certain mobs?I can not answer that at this point. But I would not mind seeing some further thoughts on what kinds of buffs you would like to see.
By the same token many players may feel that they can use that really heavy sword or armor as long as they can buff themselves, so for them, yes it will be necessary to be buffed to fight.
In the end it really will come down to how you build and grow your character. If he is generalized he will not need buffs as much, if he is really focused he will but he will probably be really good at whatever you made him for.
In this case I fear some people may make their characters such one trick ponies that at some point they may become ineffective at all. But we will see as we go and if controls need to be put in place, we will.