I recently did a position paper on LMS usability, and found the references below.
Martin 2008 was the most interesting, and highlighted efficiency of use as a big problem area in Moodle, Sakai and dotLRN.
Contact me directly if you want a copy of my paper.
Bremer, D., & Bryant, R. (2005). A Comparison of Two Learning management Systems: Moodle vs Blackboard. Paper presented at the 18th Annual Conference of the National Advisory Committee on Computing Qualifications, Tauranga, New Zealand.
Graf, S., & List, B. (2005). An Evaluation of Open Source E-Learning Platforms Stressing Adaptation Issues. Paper presented at the Fifth IEEE International Conference on Advanced Learning Technologies (ICALT), Kaohsiung, Taiwan.
Krantz, P. (2007). Evaluation of WYSIWYG
editors (2007). Retrieved 16/11/2008, 2008, from http://www.standards-schmandards.com/2007/wysiwyg-editor-test-2/
Machado, M., & Tao, E. (2007, October 10 – 13, 2007). Blackboard vs. Moodle: Comparing User Experience of Learning Management Systems. Paper presented at the 37th ASEE/IEEE Frontiers in Education Conference, Milwaukee, WI.
Martin, L., Martínez, D. R., Revilla, O., Aguilar, M. J., Santos, O. C., & Boticario, J. G. (2008, 6-7 November). Usability in e-Learning Platforms: heuristics comparison between Moodle, Sakai and dotLRN. Paper presented at the 7th European Conference on e-Learning (ECEL), Agia Napa, Cyprus. (https://adenu.ia.uned.es/web/en/system/files/openACS08_lmdrmormmjaocsjgb.pdf
Pacurar, E. G., Trigano, P., & Alupoaie, S. (2005). A QTI editor integrated into the netUniversité web portal using IMS LD. Journal of Interactive Media in Education.
Pierotti, D. (1995). Heuristic Evaluation -A System Checklist. from http://www.stcsig.org/usability/topics/articles/he-checklist.html
Rusama, M. (2006). Microsoft Word template for making Moodle questionnaires in GIFT format. from http://www.soberit.hut.fi/sprg/resources/moodle/GiftConverter.html
Storey, M.-A., Phillips, B., Maczewski, M., & Wang, M. (2002). Evaluating the usability of Web-based Learning Management Systems. Educational Technology & Society, 5(3), 91-100.
Yatskovsky, V. (2006). Microsoft Word template for making Moodle quizes. from http://www.finemetronome.com/moodle/