When it comes to choosing a phone for a unified communications (UC) solution, customers normally have a wide choice of endpoints from the manufacturer of the IP-PBX. This model of having the phones and the call server come from the same manufacturer is a longstanding tradition dating way back before my time.
Microsoft, though, has tried to break this model--sort of. When the company first launched Office Communications Server (OCS), it was built on the concept of being an open system where anyone could make phones--and indeed they could, as long as they followed Microsoft's strict reference design, the most common one being the "Tanjay" phone. A number of vendors made these phones, but the two primary companies were Polycom and LG Nortel. If you look at the pictures below, you'll see how tightly controlled the specifications were for the phones, allowing very little differentiation between manufacturers.
I believe the reason Microsoft used this model was that the company was trying to push the industry into PC-based calling and the IP phones were really a transitional device on the way to users not needing a phone at all. Also, it allowed Microsoft to have greater control over the end-to-end experience, which was important to give Lync (then OCS) credibility as an enterprise-class voice system. This model did make it easy, though, for other manufactures to make Lync "optimized" phones, and today phones are available from the two mentioned above as well as Aastra and Snom.
This model worked for a number of years, but lately a number of phone manufacturers have been developing Lync "compatible" phones. These phones follow and are certified against the Microsoft-defined 3rd Party Interoperability Program (3PIP), but also add a number of other features to add value and differentiate themselves.
Polycom's VVX is a great example of a Lync Compatible phone and offers features such as Boss/Admin capabilities; the ability to pull in SIP URI contacts (optimized phones just show Active Directory clients); support for video; touch screens; and group paging.
With the Lync-optimized phones, Microsoft dictates the features and choices. With Lync-compatible phones, the manufacturers of the phone now determine all of the advanced features. The rise of the compatible phone should create more competition and add value to the increasingly popular Lync voice environment.
The only down side of the Lync compatible phone is that it isn't a true "plug and play" peripheral. If there's an issue getting an optimized phone to work, Microsoft will take the support call and troubleshoot the issue. If there's an issue with a compatible phone, Microsoft will kick the call over to the phone manufacturer, which is reasonable since Microsoft can't be expected to know everything about every endpoint. It's a little like the PC peripheral market, where if you have an issue with a printer or non-Microsoft-branded device, you call the manufacturer, not Microsoft.
Below is a chart to help understand the differences between Lync Optimized and Lync Compatible phones. 
Optimized Compatible
I don't want to give the impression there's no value to the optimized phones. If the deploying organization is looking for a phone to simply extend the Lync experience, and if the plug-and-play capabilities combined with Microsoft support are far more important than any other feature, then by all means, use the optimized phones.
However, if the organization is looking to use the phone to enhance the Lync experience, then the features on the compatible phone, such as touch screen, on-board browser and video capabilities, make it a much better choice than the optimized phone.
Ultimately most organizations will end up choosing either Lync optimized or Lync compatible phones and end up purchasing a mix of phones within the category. The key is to understand exactly what you're buying and what you're not getting when you choose one or the other. Given the speed of innovation in the UC industry, I certainly expect to see the breadth of options for Lync-compatible phones continue to grow, giving customers more choice and more features faster than with the optimized devices--and that's good for the whole Lync ecosystem.
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