Dell's ability to serve mid to small businesses or specific teams within an organization is one big part of its moat. Many IT people directly go to Dell to order their next machines and do not think twice about it (servers, laptops, appliances etc...). They can adjust their needs through Dell's web site and order. Extremely practical, it almost has become a conditioned reflex to go see what they have to offer when new hardware is needed. As far as I know this model has not been fully replicated by others. Michael Dell has complemented this offering with additional services of higher value and margins and I think that actually reinforced this area. Himself said recently that competitors cannot serve mid to small businesses as well as they do. In many cases it is still true. They invested and are investing in cloud, storage, security, software and services. They can complement their offerings with the new kinds of devices, I do not see this area as disruptive and demands for PCs and servers will continue to grow. Margins are improving, the distribution chain has gone through a recent cycle of cost cutting and optimization, I do not think it is that easy to replicate their distribution channels.
So IMHO it is a combination of brand name - good low cost hardware, low cost on demand distribution channels, completeness of the offering and customization capabilities. This has not been replicated by anyone up to now.