Inman Square in Cambridge has a lot of great choices. I can vouch for the East Coast Grill, co-owned by Chris Schlesinger of "License to Grill" fame, and the S&S Deli, which is probably not really what you're looking for but which is worth a side trip if you've got the time. Based on a quick look at
http://www.inmansquare.com, there are a lot of newcomers at both the high end and the low end -- that neighborhood's certainly gentrified some in the last few years, but it's still between a number of great ethnic neighborhoods and manages to draw from all of them. (I'm amused to see a restaurant called EVOO -- don't know if you get your meals in 30 minutes or less, though.) Standard procedure for my friends and me back then was to head over to Inman Square and wander around until we found a place we were all in the mood for; that stretch of Cambridge St. is pretty nice and very diverse, and most places could generally accommodate smaller groups without reservations with a relatively short wait.
Central Square in Cambridge also has lots of great ethnic spots, mostly from the Middle East and Indian subcontinent, again skewing toward the low end but with a few decent "nice" spots as well. (Plus it has Carberry's, where the best cinnamon rolls on Earth are made.)
I remember Sol Azteca in Harvard Square as being a fairly decent, higher-end Mexican restaurant. Although it's not a burrito joint, I wouldn't call it rigidly traditional either, but back when I lived around there, at least, it was reliably flavorful.
Can't think of any good places around Porter Square (as you might be able to tell, my life there revolved around the T's Red Line); in Davis Square, I very much loved Gargoyle's (
http://www.gargoylesonthesquare.com/). Looks like there's a new chef, so I can't promise the standards are the same, but it looks promising.
If you all enjoy seafood, a friend suggests the No Name Restaurant on the Fish Pier (
http://tinyurl.com/9xmsy) over Legal Seafood. (The link goes to the Google Local page for the restaurant.) Although the reviews on that page are mixed, a friend says:
An IRC pal from Boston's North Shore wrote:<Joe> it's actually split in two sections, one a to-go type place, and the other half a sit-down restaurant with a more extensive menu
<bob> cool... does it suck? reviews linked from google are mixed
<Joe> I haven't been there in about 5 years, personally, but it was always my favorite seafood restaurant in town
<Joe> then legal, no-name still has the hole-in-the-wall family feel, while Legal is more upscaled and pseudo-trendy
<Joe> not that Legal is *bad*, just not as much of a real NE seafood place anymore
To be fair, some of the reviews on that Google page are pretty unhappy, so you might want to ask around among friends out there who might be more up-to-date.