SQL Alias Flexibility: When provisioning a new SharePoint farm, it is highly recommended to use an alias to connect to SQL server, as this provides for greater flexibility to move the SharePoint databases to a new server.
Crawling Recommendation: It can be advantageous to have the index server be the dedicated crawl server for a farm, as it eliminates the extra hop required to crawl content in a traditional scenario when other web servers are used. To do this, the web role must be added to the server and the farm must be configured to use a dedicated server.
Query Propagation: If the Query role resides on the same server as the Index Role, there cannot be any additional Query servers in the farm as SharePoint will not propagate the Index. For virtualized environments, this can mean fewer disk requirements for the Index corpus.
Network, network, network: Ensure your physical network provides enough capacity between the server roles in your SharePoint farm and dedicate multiple virtual network adapters to each machine, each focussed on certain types of traffic. For example, internal traffic versus traffic to users. Setup load balancing between your web frond ends to balance traffic over each of your virtual web front ends.
Database maintenance: Database maintenance is even more important in a virtual environment than a physical environment. Bill Baer authored a whitepaper (here) on measuring and reducing fragmentation of your content databases. Optimizing the speed of your databases will reduce time to fetch and store data resulting in faster response times for users of your portal.