For me, Yelp has been completely ruined by what amounts to YouTube comments.

"The server was totally rude. She put lemon in my diet coke when I asked TWICE not to do that. It was the worst experience of my life."

Foursquare seems to have less batshit insane people on it. They actually do helpful things like post the wireless password.

Yelp focuses on the "Wall of Text" type of review. Foursquare tips are one-liners.

So instead of:

"My date and I came here a few weeks ago and the initial ambience was great. We waited around 20 minutes for seats and the decor out front was..."

you get

"Try the veal. Seriously."

One is much, much more useful in the mobile context, the other is almost useless. Yelp also over-emphasizes the star-rating system (which at this point is completely irrelevant and pure noise) while Foursquare simply throw tips at you.

Overall I've found that Foursquare Explore's recommendations in NYC to be vastly superior to Yelp. I'd be sad if they go under.

combo of a lot of things, yelp's UI always seems 2 years old, feels bloated, and generally takes me longer to do anything. I use yelp on the web, foursquare on the go.