As far as gameplay i thought this would be a great addition to STO. The first page on the Star Trek Expanded Universe Wiki gives a pretty good explanation and reason for the Diligent Class. It's a good compromise, i felt the Escort with Defiant Class and the Cruisers are a pretty huge gap. It would be good to have something a bit larger with more sustainability (accommodations, sickbay, physical size) but still primarily geared for tactical and battle. In DS9 the Defiant class were always said to be this amazing warship, but it seemed small like a fighter almost.
By 2376 the Dominion War was over and Starfleet was faced with the Herculean task of rebuilding the fleet. While classes like the Akira, Intrepid, Saber, Defiant and Steamrunner were being produced at record rates to bolster Starfleet's numbers, it was decided that new designs were also needed to address gaps in functionality made apparent during the war.
The Defiant class design, for instance, demonstrated vividly the difference between a ship designed primarily for exploration and one designed primarily for combat. While the war was over and diplomatic and scientific discovery needed to resume, Starfleet Command decided it was not about to grow lax and so called for a vessel with a compact design and significant combat capabilities, like the Defiant-class, that also had the capacity to execute scientific, diplomatic and exploration missions on a small scale, when larger explorers were not required.
The Diligent class was the resulting proposal. Equipped with the latest technology, including bio-neural circuitry, new sensor suites and propulsion systems, at the expense of being somewhat larger, it represented an overall improvement over the smaller Defiant class in terms of non-combat mission versatility, while still maintaining nearly the same combat potential. However, this vessel is not designed to replace the Defiant class project, but to supplement it and other small vessels designed for short-range exploratory missions.
Typical missions for the Diligent class include border patrol, light exploration, convoy escort, and tactical interdiction. It is not suited for duty as a deep space exploratory vessel with missions longer than a year, due to its size, but it can certainly handle more local missions based out of a starbase or deep space station.