That was Awesome!, he must be Exhausted after a set.
ALWAYS know your levels throughout the room. This is especially crucial if people are eating... ruining a meal makes you a total amateur. You should have sound assistance (manager, friend) with the room - empty, half full and full - front, middle and back. Failing that, ask the customers. Know your crowd. Know your room. Your sound equipment is your instrument.
There is a huge difference between a coffeehouse and a bar. Booze adds the lack of attention, yap, yap, yap dimension. The material and levels should reflect that.
Watch people's feet to see if they are tapping. If they are not, go to the standards. If that doesn't work... you still have your day job.
Re taking requests to capture the crowd... this is an easy way to showcase what you do not know... can backfire and make you like a dummy.
Last, but not least.... remember the word "entertainer". Have fun, build up an arsenal of proven jokes, audience participation stuff, etc.. And, regardless of your contracted starts and stops, do not necessarily pack it in if the venue starts hopping with a late rush or a positive "atmosphere" change. The owner/manager will love you.
I'll offer my 2 cents on volume and consideration for your audience and allowing requests. I've been playing professionally for a long time and learned a valuable insight from my father (who I first started playing with as a kid) - in regard to volume, it depends on the venue and job you were hired for. For a coffeehouse gig/restaurant gig (which is the majority of the type I play in), people are there to enjoy a meal and conversation, with some entertainment in the 'background.' For these types of venues, the audience almost always appreciates it when they don't have to raise their voices a little more than normal conversational volume, or at least don't have to compete with the live music. We usually adjust our volume so that the patrons can still have a conversation without having to really raise their voices, and ask the manager or someone working while we're on break if the volume is OK. It's always better to have to turn up than turn down, especially when coming from the establishment. There's nothing worse than trying to eat in a restaurant or have some coffee and conversation as a patron when the entertainment is making you have to shout. As far as requests - I think they definitely can bring in your audience, although sometimes simply changing up from singing to just instrumental is a nice switch too. In the venue you're describing, singing can almost be more of a distraction while people are trying to eat or talk, as opposed to just playing. Hope this helps.
I want to re-iterate was a few have said. You are there to entertain the audience...not just sing some songs. Talk to the crowd in between songs. Find ways to get them engaged in what it going on. Sometimes you will be successful in reeling them in...other times not so much. Play alot of well known songs....Jim Croce, Seger, Skynard, Eagles...etc. Do a good job on them and record yourself playing them and listen to it. Alot of times as we play things, we come up with a certian arrangement that as we're playing it we think it should sound good....then when I listen to it after I recorded it....I though "What was I thinking!"