Almost all of the rare/powerful cards have high mana costs in some form. If they don't have high costs in one area, they'll have a significant weakness in another area.
For example, there's pretty cheap alien looking creature, I think about 4 mana or so, that has a perfect dodge and can do magic attacks anywhere on the board, albiet it's still somewhat costing to activate with I think a 3 or 4. However, he has only 1 health and some other weakess related to element.
Now, I'm not saying that those better cards won't make your deck more powerful. But they won't make your deck more powerful without the knowledge and skill to use them.
I just picked up a Biolith deck, it's pretty neat. Focuses a lot on 4-5 cost biolith creatures with only 4-6 low level creatures. It has a few interesting ways of getting extra mana to summon those creatures. I'll probably throw my extra parmetic feast in there. It's also geared heavily towards the biolith field, but that's obvious, but the neat thing is the combo the deck has with a biolith creature that fieldquakes any fields adjacent to him.
So, it's a completely different style of play from the very balanced starter deck. Heavy hitter monsters, high mana cost, so you bring them out more slowly, but if you get them on biolith fields they'll stick around for a while. It's a more slow and powerful domination of the board as opposed to the balanced spread of the starter deck.
I think I'll throw in some of those healing spells from the starter deck, the one that heals all creatures of the same element for 3, very useful in a deck that's so focused on one element. Also putting in some beguiling fogs to make sure my bioliths are always facing the right direction is useful too. But, I've got to be careful to keep the mana generation creatures, I may just drop the spells that help pull cards, although I suspect that for a slow deck like this having a larger hand is also useful. Ah decisions decisions.
One of the best aspects of this game may be its 30 card deck limit.