I'd have to agree with Bearclaw, but extend the exploration aspect to the research tree - "ooh, what cool goodies can I get NOW?
" That's part of the reason I like the Zelda and Metroid games, too - every so often you get a new weapon that does twice the damage of your old one and makes you say "I AM GOD! Nothing can stop me!" - and then pretty soon you find an enemy with ten times as many hitpoints!
