Not sure why Harvard Square gets labeled a "culinary wasteland" on this board. As a student who has lived here for 7 years now, I figured I'd explain why I really like it here.
All of these places are within a 10-15 minute walk at most, which doesn't seem too far for anyone interested enough to visit this site.
Burgers - People vary, but it seems safe to say that most really like either Bartley's or O'Sullivan's, if not both like I do. And get a shake at Bartley's - the Godzilla Western Elvis Frappe is my favorite (a whole day's calories in a cup).
Fine Dining - I've had meals at Upstairs and Rialto that were as good as almost anywhere in the city. I haven't been to Craigie Street, but it's right here too. And Chez Henri has always been good to me (plus the bar menu which is obviously well liked).
Sandwiches - Darwin's is pretty good, but I like Oxford Spa even better. And the dueling Cuban's at Montrose Spa and Chez Henri are always tasty (I'm partial to CH).
Pizza - I'm partial to noch's, but that might just be a sentimental thing. And Armando's is pretty good too, if a little inconsistent. Oggi is a nice addition as well.
Bar Food - Charlie's Kitchen is my favorite dive anywhere, and at $5.25 their double chesseburger with fries is quite the bargain (it's gone up a whopping 30 cents in 7 years). Cambridge Common is also good for buffalo tenders and their highly regarded sweet potato fries. And the beer selection at both places is great too.
Ice Cream - Again people differ, but Herrell's and Lizzy's are both pretty good (especially the Charles River Crunch at Lizzy's). And you can still pick up Toscanini in nearby grocery stores. And Berry Line is pretty good for Pinkberry-style fro-yo.
"Date Places" - West Side Lounge and OM both have good food, a fun vibe and great drinks. Cambridge 1 is always reliable. Garden on the Cellar is a little further away, but surprisingly good. Small Plates seems like a nice addition, though I haven't been yet.
Bakeries - Rosie's up on Mass Ave for brownies, Hi Rise for pretty much everything else.
"Ethnic Food" - Some of the Thai and Indian places around the square rightly get some criticism. But Tamarind Bay is pretty good, and the food court in the Porter Exchange is really a hidden gem.
Other favorites - Burdick's for hot chocolate, Formaggio for the best cheese selection in town. Cardullo's for a great chocolate selection and other treats. Brunch at Henrietta's Table. Chinese delivery from Zoe's.
I'm sure there are others I forgot, but hopefully this is helpful for anyone looking to eat well in the neighborhood.