For one, they have tanks with unique dimensions. I want a 60-f so bad, but it's $110 for an 8.9 gallon tank! I love that it's 2 ft long, but shallow, so it's wider than it is tall. If Mr. Aqua made a tank with those dimensions (24" x 12" x 6"?) I'd be all over it. I really don't care about the glass quality - I have a 12 gallon long by Mr. Aqua that suits me just fine even though it's not the best glass there is. Not a priority for me. Do!Aqua has the 60-f in lower quality glass but of course they're out of stock :P
To me, ADA means top quality glass plus unique dimensions.
Eheim pimp #255
75 gallon semi-aggressive community tank with pressurized CO2, 8.6 gallon cherry shrimp tank
40 breeder reef
12G Rimless Club