Troy has a good detailed description of some common value/comparison issues. I'll hit briefly on some of the main factors we observe and compare when formulating our value opinion. Keep in mind we are searching for recent sales with similar attributes.
Subject Property
* Location - is it on a busy street, private gated community, end of a culdesac, backing high traffic street or freeway, airport noise, across main neighborhood boundary where the market appeal changes, in highly rated school district, etc.
* Site (Lot) Size - typical size for the area or atypically large or small and how does that affect potential buyer appeal, or strange shape lot (pie shape, etc) which decreases its' utility (use).
* Design/style - certain neighborhoods may show preferences for specific home styles/designs, I have some markets near me where Craftsman (arts and crafts) style houses are highly desirable and command a premium, I also have seen many relatively recent new builds and rehabs which are modern style and are getting premiums based on that. This depends on the neighborhood and what buyers appear to be showing desire for.
* Quality - Is the house modernized with current materials and finishes, are they higher end upgrades, materials and finishing (solid surface countertops-Quartz, etc, custom solid wood cabinets with soft-close drawers/doors, built-in SubZero refrigerator/freezer, 48 in Wolf range, wine/beverage fridge, large center island with prep sink, solid wood or good quality engineered wood flooring, etc) or does it have outdated or minimal upgraded finishes (Formica countertops and cabinets, tile vanities, vinyl roll flooring, basic carpeting, no major upgrading, etc).
* Age - similar in age to the surrounding homes or significantly newer or older
* Condition - has the house been fully remodeled recently, partially remodeled, or just basic maintenance and updating as needed
* Bedroom/Bathroom count - # of each and is that typical for the area or is it a 2/1 but most buyers pay more for a 3rd bedroom and/or 2nd bathroom, or does it have a "superadequacy" like 6 bedrooms in an area that typically has 2-3 bedrooms
* GLA (gross living area) - size of living area, again is it typical for properties in the area or under or over built. In some areas, buyers want larger houses to accommodate their growing family and therefore, may have additional appeal for larger homes (1,200sf vs 2,500sf), or they want additional space for a home office or gym, etc.
* Heating/Cooling - central heat and air or wall furnace and no AC, if the house is in a community close to the ocean, AC may not be a big deal, but in hot environments, it may be seen as a necessity and this may be reflected in prices.
* Garage/Carports - # of each, some areas have a premium for garages since the weather will affect their utility. Basically, if it snows or rains heavily in your area, you would probably want an enclosed garage and preferably attached to the house with direct access, and buyers may pay more for that.
* Yard amenities, etc - in-ground pool/spa, covered patio, built-in BBQ's, elaborate yard landscaping and upgrades, etc.
Those are many characteristics that we look at when evaluating a home, and then we are looking for recent sales that, hopefully, have many of those attributes, or we piece together different sales which have some of those attributes, to reflect the important characteristics of the Subject Property in the market. It is a combination of art and science.