I am 28 and have lived in Costa Rica for five years. My best friend lives in Lanikai, Hawaii. It's funny that these two destinations are so often mentioned in the same breath, as they couldn't be more different. Having visited Hawaii several times and enjoyed discussing the pros and cons of each with my friend (who visited me here in CR four times) I think I have a fairly educated, objective opinion on both. My friend and I both work in tourism.

Hawaii is a gazillion times more developed for tourism than Costa Rica. The highways, the 747s flying INTER-friggin-island, the giant resorts, the condo megaplexes, the tourist trappy bars, restaurants and 'cultural events' reflect years of experience in the tourism industry. It's the best and worst of a mature tourism industry. When I'm there I am in awe of the cleanliness of the parks, the neatness of the public areas and the amazing beauty. Before I get rambling, here's my pro/con list as best I can remember:

Hawaii Pros:

Easy to get to many islands, city to city
An ulta modern city
High end nightlife, dining, tours (helicopter), golf
Fabulous snorkeling, surf, scuba
Perfect weather most of the year
Honest police force, safe
Immaculte parks
US dollar currency (pro and con)
English spoken

Hawaii Cons:
Incredibly pricy
Remote (a long haul from NYC)
Very, very touristy...some of those annoying, overweight loud and complainy kind.
Little remaining local flavor, culture
Crowds
Minimal wildlife
EXspensive!!!

Costa Rica Pros:
Still some 'undiscovered' spots
Warm Pacific and Caribbean waters (2 coasts!)
Great surf
Some cool local culture (but you need to search) in the countryside
Spanish spoken (if you like adventure)
Nearly perfect temperature all year, dry for 7 months
In general, cooler 'travelers' here, less Hawaiian shirt tourists
Wide variety of climates, flora, fauna, animals, birds
Proximity (a few hours from Miami, Houston)

Cons:
Not as safe as Hawaii (I personally like less cops), so be more aware of surroundings
Lousy snorkel, scuba compared to Hawaii
Bad roads, drivers, long travel times
Lower quality of service
Can be pricy in places despite lower standards
Funky currency (you need to learn it quickly or get ripped off) and difficult use of debit cards
Poor infrastructure (cel/telephone, internet)
Becoming too popular, peak season is nutz