I am here not to defend for Jong Suk but just to friendly share my opinion.
I agreed that his acting style is kinda the same until While You Are Sleeping, but looking back, most of his dramas are melo *plus* written by the same writer. She knew his style of acting so personally she did him tailor-made. And regarding to his speech patterns & voice tone, he has improved here and there.. I hope that you could spend a bit of your time on Hymn Of Death, so may be you could see him a bit different. It is the only one (too bad, it's short) drama that I can see him in a new light, it's the only one drama I admit that his acting skill has reached another step, after Dr Stranger (and Hot Young Blood movie). It took me almost 4 years to witness his improvement so I really love it (I dont mean that his acting was the same in 4 year long. Except W he performed poorly, he did improve a bit here and there, detailed one on voice of tone, micro expression etc but those cahnges are unnoticeable easily)
And to tell you the truth, what you see LJS here as LJS vs CEH, is true. Mature, calm, warm CEH is CEH and cute, dorky, adorable CEH is really LJS - that's how he is in real life (same with Jung Jae Chan in While You Are Sleeping) However, mature, thoughtful, considering and self-low esteem LJS, those sides of him are not able to see unless you read his interview and analyze his action sequence.
It's totally ok if you and other drama-lovers dont see him the way as I do, because I have always truely hoped he could experience much more and build up his skills much wider and stronger. He needs to change his image, whether sharper, uglier etc in order to reach further remarkable milestone, especially after discharge.