As others have mentioned, a west-backing home does present somne challenges in the Midwest, including Kansas City. Use of a deck or patio on the west side of a home in the summer months is considerably impacted by both heat and glare. If you have sensitive eyes or sunburn easily you might want to consider your unprotected backyard areas to be of little value to you from June through early September. Of course, if you find located a west-backing home that is otherwise perfect, you can always cover your deck or patio with a retracting awning, overhead arbor or something similar. If you are from a different portion of the country, do not underestimate the intensity of the sun in a Kansas City summer. This would be particularily important if you plan to entertain very often.
Regarding the sloped backyard, it is much better to have slopes extending away from your home than towards it. I assume, as have those responding before me, that the slope is fairly rapid and downhill from the house to the back of the lot. One of the largest disadvantage is that it limits placement of items you may want to include in your yard. This would include children's play equipment, a pool and even garden areas. The sloped lot does provide for either a walk-out basement or one with daylight windows. A steeply sloped lot can also create a much more difficult task when it is time to mow the grass. On the positive side, some of the most beautifly backyards I've witnesses were sloped or otherwise difficult to maintain. The homeowners had terraced portions of the lot with rock or timber retaining walls and working their preferences of plantings and landscape into them.
Depending upon how severe the exposure to the western sun or how steep the backyard is, either issue can make it more difficult to resale your home when and if that time comes. Remember, what is an issue to one prospective buyer is a benefir or joy to another. Best of luck.