Two ideas I like best are:
1. Klingon-Romulan alliance, or at least alliances between certain factions within those empires. We consistantly see strife between the different houses of Klingons (Duras) and different factions in the Romulan Empire (Reunification, Nemisis). I like the explaination that the lack of Klingon bumps in TOS is because at least a house, or leading house of the Klingon empire had allied itself with the Romulans or a Romulan faction and it was invouge to look more Romulan. History has similar instances, particularly old England where the leadership acted and spoke more French. I think the alliance theory explains a lot and does not have to limit Romulan power.
2. A second explaination could be that the "Klingon design" is just a standard shape of spacecraft in TOS and that the Federation is the odd man out with its designs. Even the ringed version of the Enterprise (briefly seen in The Motion Picture) had a similar design to "Klingon" ships. Both Reliant and Voyager have similarities to the D-7 design. If you think about it. Around the world, most naval vessels have a similar shape. One nation does not use all catermarans, and one mono-hull vessels. Infact, I would not be surprised to see a D-7esk ship with Federation markings.