The other guys responded well to the suspension. Not sure what year you are talking about but from my standpoint (own a 05 WR250F) regarding power:
-the bike come with the POTENTIAL to have tons of power. However, you have to make some free modifications (mostly easy) to get ahold of the power available.
1/ cut the gray wire (details available on this site)
2/ remove the baffle from the exhaust
3/ cut out the air box and put vents in ($8) to provide more air to the air filter.
4/ replace the air filter ($15-25 depending on the kind you want) with a better one (I have a Twin Air).
5/LAST and MOST IMPORTANT: cut the throttle stop down so you can actually use all of your throttle. Yamaha puts a big throttle stop on the bike for some reason, have to get that off asap.
If you only want to do free things, adjust the throttle stop, cut the gray wire, and remove the baffle. This will increase performance and power.