So i'm looking at putting together a MicroATX system, and i'm not seeing much in the way of cases for such. The list of case recommendations breaks down into big boxes, horizontal boxes, and tiny boxes. Are there not enough mid-size cases to justify breaking them out separately? It turns out the horizontal boxes are all reviewed for HTPC use, and so it seems none would be appropriate for a desktop. Since there isn't an "Editor's Choice" for an mATX desktop, what would be a good choice?

In North America most desktop computers use floor standing tower style system unit cases. Two benefits are that desktop space is not wasted by the system unit and the noise of the system unit is further from the ears. Floor standing tower style system unit cases need not be any smaller the standard ATX case size which fits under a standard computer table or desk.

There is one SilverStone Temjin TJ08-E: MicroATX Evolved, in my personal experience i find the frontal fan too noisy even at low settings, i had to undervolt it to 600rpm where its acceptable, but there is a newer similar version just with 2x120mm fan on the front, so it should give you much more options to chose if you decided to get better fans, Silverstone PS-07.

Depending on your needs and if you appreciate smaller, you can go into Mini ITX factor, you can still build a fairly power PC with the options available today, Cases like LIAN LI PC-Q11B that should allos you to have a fully sized ATX PSU a mini itx mobo, a dedicated GPU (limited to 260mm if i recall correctly), and so on. Check New Lian Li PC- Q11 itx case : Specification & Feature for builds and ideas.

If you like something really small, dont need a quad or optical drive or dedicated GPU, you could build on really small enclosure, like for example,
CASE/MOBO/PSU: M350-INTEL-DH61AG-19V-160W KIT
CPU: Intel Core I3-2120T (low profile cooler to allow the mechanical hdd bracket)
MEMORY: CORSAIR 8GB (2 x 4GB) 204-Pin DDR3 SO-DIMM DDR3 1333 Laptop Memory Model CMSO8GX3M2A1333C9
SSD OS/APP: Intel 310 80GB (samsung has faster 128gb msata ssd but seems only available as OEM)
HDD STORAGE: Western Digital 1 TB Scorpio Blue SATA II 5400 RPM

LIAN LI V351 W 10.98; H 10.31; D 14.69; (ITX? & MATX)
BGEARS BG01206 W 11.20; H 9.00; D 14.70; (ONLY MATX)
APEVIA X-QPACK II W 11.25; H 9.00; D 14.50; (3 WINDOWS) (ONLY MATX)
APEVIA X-QPACK W 11.2; H 9.00; D 13.80; (3 WINDOWS) (ONLY MATX)
LIAN LI PC-Q07 W 7.60; H 11.02; D 8.19; (ONLY ITX)
LIAN LI PC-Q08 W 8.94; H 10.70; D 13.58; (ONLY ITX)
ARK MINI ITX PI-01 W 8.00; H 7.00; D 12.75;
SUGO SG05 W 8.74; H 6.93; D 10.87; (ONLY ITX)
SUGO SG06 W 8.66; H 6.97; D 11.26; (ONLY ITX)
SUGO SG07 W 8.74; H 7.48; D 13.78; (ONLY ITX)
SUGO SG02F W 10.6; H 8.30; D 15.50; (ONLY MATX)
SUGO SG03B-F W 7.87; H 14.17; D 12.28; (ONLY MATX)
APEX MI-100 W 8.70; H; 5.10; D 11.80; (ONLY ITX)
APEX MI-008 W 8.70; H; 5.10; D 11.80; (ONLY ITX)
LIAN LI PC-Q09 W 7.85; H 3.86; D 7.85; (ONLY ITX)
ANTEC NSK1380 W 10.60; H 7.90; D 13.20; (ONLY MATX)
ANTEC NSK 3480 W 7.87; H 13.78; D 13.98; (ONLY MATX)
ANTEC NSK 3300 W 7.80; H 13.80; D 13.80; (ONLY MATX)
ANTEC NSK 4400 W 7.80; H 16.50; D 16.60; (ATX & MATX)
ANTEC NSK 4480 W 7.80; H 16.50; D 16.60; (MATX & ATX)
LIAN LI PC A03 W 7.40; H 14.49; D 17.60; (ITX & MATX)
LIAN LI A05 W 8.27; H 15.00; D 19.29; (MATX & ATX)
LIAN LI PC-A06F W 7.36; H 14.76; D 19.29; (MATX & ATX)
ANTEC SOLO W 8.10; H 17.00; D 18.50; (ITX, MATX & ATX)
CENTURION 590 W 8.27; H 17.32; D 20.08; (MATX & ATX)
ANTEC ELITE 341 W 7.28; H 14.37; D 15.75; (ONLY MATX)
SHUTTLE SG41J1 W 8.46; H 7.48; D 12.79;
LAN-GEAR.COM W 11.4" X H 8.6" X D 14.6" (ONLY MATX)