The Bodies Inside
Think about who will use the tent, then look at floor space, head space and the sleeping configuration map. If you are tall, consider only tents with above-average length and head space.
On extended backpacks, 6 extra inches of space may be worth carrying. If you are a weekend backpacker, a smaller tent is adequate.
Identify Your Preferences
Note tent pitching configurations. Handily, many backpacking tents can be pitched with only rain fly and footprint. Some use trekking poles instead of tent poles.
The Manufacturer and Vendor
Some companies steadfastly support their products and will repair or replace defective parts. Some even drop-ship to wherever you are. Regional chains often sell a quality house brand tent for a little less than major manufacturers' analogue.
Tent, tarpaulin and bivvy sack manufacturers include: Big Agnes (tents), MSR (all), Sierra Designs (all), The North Face (tents, bivvies), Eastern Mountain Sports (EMS) (tents), Mountain Hardware (tents, tarps), Black Diamond (all), Marmot (tents, bivvies), NEMO Equipment (tents), Kelty (tents, tarps), Integral Designs (all), Rab (tents, bivvies), Montbell (tents), TarpTent (tarps, tents), Golight (tents), Gossamer Gear (tents, bivvies), Six Moon Designs (tents, bivvies), Oware (bivvies), Bozeman Mountain Works (bivvies, tarps), Granite Gear (bivvies, tarps)
Backpacking shelters balance durability with weight. Notice material, seam construction, waterproof coating, bathtub floor, zippers, custom footprint, poles, nails and guy line fasteners. Compare both the manufacturer's description of components and what owners say.
Most backpackers agree that a two-door tent makes tentmates tolerable. Ventilation also is important. The last consideration is what extras are included: gear lofts, side pockets, flashlight loops and the like.
Take the "love it" factor into consideration. This will be your home when you're exhausted, exhilarated, sore and hungry. Since it's a compressed space, liking your digs will help your mood.