A typical but potentially major mistake in the design of a Lync solution can be summarized in two words: human factor.
It is typical among technical decision makers, especially those from hi-tech firms, to put excessive bias on technological aspects while overlooking or forgetting the end users. Specifically, we refer to the following points:
Expectations: The ability for your solution to meet their demands/requirements
Acceptance: Involving users in the decision-making process, and allowing for their opinions to influence the final solution
Experience: Envisioning a simple and effective solution that can suit people with diverse skills and culture
Knowledge: Making people aware of your new UC solution, and training them to maximize their productivity and effective communication
Technology definitely matters, and if your infrastructure fails to deliver a consistently good Quality of Experience, the users will abandon it, which will render your investment void...