boo boo said:
Leaving the cover on your kettle for the boil COULD and I say COULD cause your brew to have certain undesireable flavors.
Well, this is a rather controversial topic... where's that pic of an open can of worms?
The issue that some people are worried about is the buildup of DMS, which lends an off-flavor to beer (described as cooked vegetable or corn). DMS is formed whenever the wort is above 140 degrees, IIRC, but since it is very volatile, it is easily driven away by the boiling processs. So, leave the lid off, DMS is driven away, no problems.
However, I've heard interviews with the people from Breiss Malt Extract on Basic Brewing Radio, and what they have indicated is that most of the DMS has been already been driven off during the manufacturing process of the extract, so it's mostly a non-issue for extract brewers. Remember, they're basically making a huge, unhopped wort and then concentrating it, so a lot of these issues have already been taken care of.
So, no worries... but I still worry a
little, can't help it
The compromise that I made when I was doing extract on a wimpy electric stove was to leave the lid partially on, to try and minimize any condensation being returned to the brewpot, but not to worry too much about it. I've never detected any DMS in my brews (not that I'm 100% sure I'd be able to pick it out). I wouldn't leave the lid on completely, but that's more because you are BEGGING for a boilover if you do. In any case, if you have the lid on at all, you can't leave the brewpot, it'll start boiling over like crazy if you aren't careful.