Water loss

Water surface aggitation and skimmer will take care of gas exchange. If the lights are elevated and have a fan heat shouldn't be a problem.

 

It depends on the physical configuration of course. For example, if the hood is enclosed on all 5 sides (4 sides + top) you'll get trapped air that surface agitation won't have fresh air to exchange with. Like Jed says though, if your skimmer is big enough it'll take care of the gas exchange for the most part.

You'll also have the possibility of trapped heat unless you fan it out somehow (a lot of folks put fans in the back or side of the hood for these reasons). A lot of hoods don't have a back side which gets around a lot of this. I've seen them both ways.

The fan for the lights, that depends on what you mean. My SunDial lights, for example, have 2 fans, but they're removing heat from within the fixture, not from below the fixture where some of this heat is being transferred to the water. And if the hood is enclosed all around, those same fans are just moving the heat around, not getting rid of it. In that case, those fans aren't really doing anything for you. So you'll want a fan between the lights and the water separate from the fans that are part of the light fixture. Having just a fan in your lights within a hood won't take care of the heat issue.

I'm also saying this with my Houston POV where it is pretty much always hot.

 

I see tanks all the time with out hoods. I dont get why the fish dont jump out. I had a tank once that had a hood and I found my hawkfish sitting on the ground in the morning. He jumped out the back (vented area). I dunno, guess you are all lucky, or you dont have wrasses or hawkfish.