Some sugars will create off flavors, usually cidery.. this is usually the case with table sugar. Corn sugar (priming sugar) probably won't have this effect but it will make your beer more watery as it will all ferment into alcohol without adding any body or flavor. This is why most suggest that you just use additional extract or, if you're an all grain brewer, more grains. Honey is another option but it can have a thinning effect as well that may or may not be desireable. It's also pricey and does lend a flavor of sorts to the brew - some like it, I'm not really too fond of it but that's just me.
Probably the biggest concern with making a big beer is that you balance it otherwise it will have a strong alcohol taste and be thin and watery. Think Steel Reserve or Natural Ice... so, to avoid this, you need to be sure to balance the alcohol with grains (or extract) and/or hops. That way you can avoid that Steel Reserve nastiness and have a strong IPA or Imperial Stout instead.
Have fun, but don't go overboard. I will say that a 6.2% abv beer is noticably stronger than the typical swill - you wouldn't think so but it is.. at least in my experience.