The metadata discussion has to start by identifying the core terms that drive the business needs and it has to start early in the DAM design process...the earlier the better. Collaborate with key business stakeholders in the organization to build consensus and look to this effort to ultimately shape the rules for users, data creation, modification and administration.
In general, I would advocate a "less is more" approach to metadata with as little free text data entry entry as is possible. This may not aways be important in a small organization but it can be a huge issue in a larger organization. It's all about consistency and if you can't control the quality of the data into the DAM you will diminish its overall value back to the organization. In visual media, the visuals in search results will do most of the talking....you need to make sure that potent search terms and keywords are getting the creative's eyes to the right images. Take the user perspective into the design process, how will they search? What will they be looking for? Is it better to have 20 great results in a search or 60 that are just ok and require additional digging? Make it easy.
What kind of metadata do you need to capture? When? How? By who or what? Lastly. what will the metadata do? Will it be synchronized across different platforms? How? Will it be driving automation within the workflow? All of these questions need to be incorporated early into the overall DAM design plan. I would't assume you can copy or buy a metadata schema and just plug and play. I would also take a long hard look at any legacy data and try to weigh its potential value. What is its trustworthiness and how portable will it be into a new system? Is this an opportunity to filter and clean legacy data before migrating old problems to a new environment? OR, is this the chance to abandon the legacy data and all its "baggage" and start afresh?
The ROI will come from the combined excellence of a metadata and technical solution...you need both. Without consistent and accurate metadata the ability to succeed is significantly jeopardized. Ultimately, if the metadata fails for your business needs it won't matter how good the GUI looks, how fast the performance or how effective the storage solution.
Metadata matters.