In the Shadow of Linguistic Giants
Central America is a sprawling, gradually narrowing istmus from north to south, where it begins at the bottom of the Yucatán peninsula and narrows as it turns southeast, tapering into the isthmus of Panamá, famous for its canal connecting the western seas with the Pacific. Due to its sprawling geography and length, and its relatively smaller populations (and great poverty) its six Spanish-speaking countries find themselves pinned between three giants. To the north, the region is overshadowed in many ways by Mexico, with its large population, cultural, political and other influences. To the east, and across the water, Central America's Caribbean coastal areas are greatly influenced culturally, gastronomically and linguistically by the Caribbean (including English-speaking Jamaica which exerts a cultural pull on many of the concentrations of African populations along those coasts). To the South, of course, the entire continent of South America exerts some influence, but none so much as Colombia. Panamá was "invented" by the US in order to build the canal, so, prior to that time, Panamá had been a departamento (province) of Colombia and politically part of South America.
Beginning in the north, the Spanish of Guatemala resembles the Spanish of southern Mexico (Chiapas and Yucatán) in many ways, but based on my experience, Guatemalans tend to be somewhat more soft spoken. And, unlike Mexicans, they do not say ¿Mande? when they want you to repeat something!
The Hondurans, Nicaraguans and Salvadorans are difficult to tell apart linguistically -- except when one hears them speak of their national foods or holidays. They tend to be quite competitive among themselves and sometimes this takes on a nasty flavor. The ups and downs of their economies and political regimes have at times made servants of their respective populations and, when they travel to a neighboring country to work, they are often treated with contempt. Of the three, Honduras is the poorest, El Salvador the one that has seen the devastations of civil war and Nicaragua, a relative giant among the group, has witnessed the Sandinista Revolution (1979) and its after shocks. El Salvador, located on the Pacific coast, does not experience the linguistic pressures or cultural influences of the Caribbean as does, for instance, Honduras. Yet in all three countries, there is a tendency toward nasality and their accents tend to be identified by default: not Caribbean and not Mexican, with some of the clarity of the Mexican but the nasality of the Caribbean.
Costa Rica stands out in Central America for many reasons (among them being a stable civil -- non-military -- government). Linguistically, Costa Ricans speak the clearest Spanish in Central America, possibly tying with the Guatemalans, but the Costa Ricans tend to speak a bit more energetically.
Finally, Panamá. Linguistically, Panamá is strongly influenced by the Caribbean, being an extension, geographically, of the Caribbean coastal provinces of Colombia. The Spanish of Panamá has a definite nasal quality to it, and although they tend not to omit the S at the end of syllables, they will often soften an intervocalic consonant to the point where it will nearly be unheard. This happens in Spain as well, particularly in the slang of young people and among the gypsies.