The first relates to the fact that while synthesizers were able to store data associated with sounds for later recall since the revolutionary Prophet-5, they were only able to do so in limited numbers due to onboard memory being so expensive. Even into the 1980's any machine capable of storing 32 or even 64 patches was considered well spec'd. This is a far cry from the situation we have now where instruments routinely come bundled with hundreds if not thousands of presets, particularly in software.
Very quickly sound designers ran out of room so to be able to 'offline' additional patches elsewhere was a godsend, even if was only to a second machine of the same type. Musicians loved this idea as it also allowed their instruments to be loaded with a particular sound set quickly and easily before a gig, something that was not possible before without constant and laborious re-programming or configuration.
The second major restriction of the day was that editing this new breed of digital instrument was a right royal pain in the proverbial and remains to this day the single most contentious characteristic of these machines. Having to navigate through button driven, hierarchical menu systems with the only visual feedback being a small two-line display caused much consternation particularly when the synth in question came with an unfamiliar synthesis method (no prizes for guessing which one!!).
Interestingly this interface design was the inevitable trade-off between cost to build and the functionality offered. In many cases some machines may never have been built at all if an individual knob or slider had to be provisioned for every single parameter. Front panel space further became a premium as MIDI also allowed instruments to be form-factored into a 19-inch rack and triggered with an external controller. SysEx in it's simplest scenario allowed for 'bulk dumping' between similar machines but more importantly it allowed emerging 8-bit computers and clever programmers to create an on-screen user interface that could display the synths architecture and all of it's editable parameters in one view. Once in the computer realm the number of patches that could be stored and edited became almost limitless. A more hardware based attempt at providing individual controls was arguably developed by Roland with their PG controllers offered as options on their various synths. At its heart however SysEx was predominantly the messaging used under the covers.
Compact, desktop, analog, semi-modular - whats not to like?