The logic execution speed is a key differentiator. The PLC has been designed to meet the high-speed applications demands that require scan rates of 10 milliseconds or less, including operations involving motion control, interlocking, high-speed or control of motors and drives. It is necessary fast scan rates to be able to effectively control these devices.
The DCS does not have to be that quick most of the time. The regulatory control loops scan normally in the 100 to 500 millisecond range. It should be detrimental to have executed of control logic any faster possibly causing excessive wear on final control elements such as valves, resulting in premature maintenance and process issues.
The insurance policy of sorts, an extra cost for redundancy, may be well worth it in the case of the typical DCS system, where high availability is mission critical. It is often not cost justified to make a PLC system fully redundant.
Taking the PLC system offline to make engineering changes and configuration may have less impact, since the platform is not running continuously or because the process can be restarted easily. In contrast, engineering changes and configuration changes to the DCS system are done online, while the process is running virtually non-stop. Many process applications may only shut down once or twice a year for scheduled maintenance, while others, such as a blast furnace, are planned to stay on-line continuously for 5-7 years.
The analog control issue is important, but confusing. DCS was designed originally for delivering analog control, but to say the DCS has a lock on the analog control market reiterates the problem with traditional thinking. The PLC is capable of delivering simple to complex PID control, but the DCS is clearly the choice for applications with a large amount of advanced analog control, including model predictive control, cascade loops, ratio, and feed forward loops.
These are advanced solutions for process control that are driven as much or more by a vendor's domain knowledge and experience as the different platform capabilities. It's that expertise and heritage, which comes from meeting customer requirements for decades, that differentiates the systems, not the technology. If a company can not explain to you how their feed forward loop works, for example, they probably do not offer true "DCS" control.