I agree completely with WalterKovacs' assessment that the kinds of effects that a power has, and the defence it targets, is correlated with its keyword/s. This correlation of effect, defence and keyword is not wholly complete across all powers; but there is a correlation nonetheless.
What we have seen since PHB1 is a massive expansion in the classes and the combat mechanics used by those classes, including related powers and feats. At the same time, WotC has endeavoured to make as many character builds as possible as viable in as many circumstances as possible. Hence, we have illogical things such as pyromancers that can burn fire elementals. Whether this is because of some deep-seated desire to appeal to the punters, or to ensure that organised play functions no matter what is brought to the table, or some other reason, it is, IMHO, the crux of the problem.
The problem should not exist in home games. Players should develop their character concepts in conjunction with the DM so that everyone's happy. If you want to bring a pyromancer into my Against-the-Primordials-themed campaign, I have to warn you at the start that about one quarter of the bad guys you're fighting will be effectively immune to your attacks. A fire attack does not harm a creature literally made of fire; my campaign, my rules, RAW be damned. Equally, if I as a player have this cool idea for a goth-themed dread necromancer, I'd better hope that my DM isn't running a Ravenloft campaign full of necrotic-resistant undead - unless I want to side with the bad guys, of course.
There is no balance between the keyword-applied powers. That's a given. To then suggest that your character concept has been nerfed because suddenly not every power that it might have had access to won't get all the funky benefits of your "one chosen" keyword sounds to me like... Talk to your DM. If he or she is OK with a pyromancer applying the fire keyword to a power like Hold Monster with (melted) cheese, your problem is solved. Calling foul over WotC's latest rule change... well, I have two young children, you get where I'm going.
Cheers, Al'Kelhar