Two more factors that made the WHL Wallace - Prentiss - Hurlbut Line a strong position:
The number of Federal troops engaged (numbers taken from DW Reed, Pages 91-101) with WHL Wallace providing 6000 (after separating out 13th Missouri and three Illinois regiments from 8400); Prentiss (estimated to have arrived with 500 or so; but augmented by the 23rd Missouri (575)); and Hurlbut (minus Veatch) contributing about 5000... for a total (at its peak strength) of 12,075 men.
Capable leaders. This became apparent to me when I encountered the performance of Major Ezra Taylor (working for Sherman and McClernand as Chief of Artillery on the Western side of the Battlefield.) Because the WPH Line benefited from having two competent directors of artillery: Colonel James McPherson and Major John S. Cavender. The man responsible for "sounding the alarm" at 4:55am (Major James Powell) also contributed to the effort, for a while; BGen Stephen Hurlbut may not have been the most dynamic leader, but he was steady, and most of his decisions were sound... and he benefited from having a bold, dynamic leader as part of his 4th Division: BGen Jacob Lauman. There were several meritorious Colonels of Regiments, two of which come to mind are William T. Shaw (14th Iowa) and Hugh Reed (44th Indiana). Last but not least, the team of WHL Wallace and Benjamin Prentiss. Until Wallace showed up in person (after placing his artillery), Prentiss believed that the 2nd Division was under the direction of MGen C.F. Smith; so the arrival of his old friend from the Mexican War would have been a pleasant surprise. And WHL Wallace admired Prentiss: there is no doubt the two of them worked well together, sharing infantry and artillery, and ideas on how best to fight their position. And the two Mexican War veterans appear to have worked well with Stephen Hurlbut (because the sharing of infantrymen and artillery pieces included the 4th Division.)
Will add more factors, as they come to mind...
References: http://babel.hathitrust.org/cgi/pt?id=hvd.32044011616513;view=1up;seq=105 DW Reed from HathiTrust
http://archive.org/details/lifelettgeneral00wallrich Life and Letters of WHL Wallace from archive.org