Why is too much glucose harmful?
High intracellular glucose.

Affects: all cells that do not depend on insulin to take in glucose. Examples: neurons [1], kidney cells, retina cells.

Causes: high extracellular glucose (in most cases hyperglycemia)

Effects: promoting necrotic cell death through $H_2O_2$ (peroxide) formation, which may participate in the development of diabetic vasculopathies and associated disease [2].



High extracellular glucose.

Affects: extracellular matrix, cardiac muscle

Causes: lack of / low levels of insulin, insulin resistance

Effects: enhances collagen production thus affecting kidney by thickening glomerular basal membrane [3, 4]. It promotes cardiomyocyte metabolic stress and altered glucose handling [5]. It is also a potential mechanism involved in cardiac glycogen accumulation [5].



Diabetes mellitus involves:


high extracellular glucose, leading to:
high intracellular glucose in cells that do not depend on insulin to uptake glucose;
low intracellular glucose in cells that depend on insulin (muscle and fat tissue).




References:


David R. Tomlinson and Natalie J. Gardiner. Glucose neurotoxicity (2008). Available from http://didattica.uniroma2.it/assets/uploads/corsi/140868/Glucose_Neurotoxicity_-NATURE_Reviews-Neuro,_Jan_2008.pdf (accessed 27.07.2014)
RnCeus.com. Glucose Homeostasis (2012). Available from http://www.rnceus.com/dmeds/glucose.html (accessed 27.07.2014)
Takeuchi A, Throckmorton DC, Brogden AP, Yoshizawa N, Rasmussen H, Kashgarian M. Periodic high extracellular glucose enhances production of collagens III and IV by mesangial cells. Am. J. Physiol. 1995 Jan;268(1 Pt 2):F13-9. PubMed PMID: 7840239. 
Ayo SH, Radnik RA, Garoni JA, Glass WF, Kreisberg JI. High glucose causes an increase in extracellular matrix proteins in cultured mesangial cells. Am. J. Pathol. 1990 Jun;136(6):1339-48. PubMed PMID: 2356864. 
Kimberley M. Mellor , Upasna Varma , David I. Stapleton , Lea M. D. Delbridge. Cardiomyocyte glycophagy is regulated by insulin and exposure to high extracellular glucose. American Journal of Physiology - Heart and Circulatory PhysiologyPublished 15 April 2014Vol. 306no. H1240-H1245DOI: 10.1152/ajpheart.00059.2014