Star Trek Online at launch. The game's division of ship tiers into customizable batches is no doubt part of the reason, as well. But that doesn't mean that the developers aren't well aware of the gaps in ship classes, and that's the reasoning behind the latest poll on the official site, asking players which ship design they'd like to see enter the game next, quite possibly as a lower-tier ship.
Of the classes available, four are past or future designs for the ubiquitous Enterprise lineage, including the often-seen Excelsior class and the rarely-seen Ambassador class. There's also the far-future Enterprise-J design and the far-past NX design, both seen in Star Trek: Enterprise. The other two classes are somewhat less flashy but somewhat more notable omissions -- the Nebula class, a frequently-seen workhorse design that resembles a pared-down Galaxy class, and the Oberth class, a general-purpose medical and scientific vessel from the same area. The Excelsior class is currently in the lead, so Star Trek Online fans are encouraged to vote for their favorites.