As I try to nail down exactly what I would consider a modern gentleman to be, I keep getting dangerously close to clasifications I very much want to avoid. They've become the sort of borders of my definition. The idea of a gentleman lies in the space between the following characters. (In an opposite Jeff Foxworthy (You might not be a gentleman if...) style
Metrosexual:(aka Pretty Boy) A gentleman takes care of himself. He takes pride in his appearance and is well appointed. But he maintains his masculinity. I'll put it this way, a woman might desire to put on an article of a gentleman's clothing, but never the other way around. For instance, no skinny jeans, fancy scarves or cute anything. A gentleman might go in for a steam and a rubdown. But he wouldn't be caught dead getting a facial or having his hair dyed.
Dandy:(aka Preppy) Specific to things related to boys, and manners befitting a prat. Much of the exploits of a gentleman might be easily confused with that of a prep, (Sailing, tennis, cocktails, dinner parties, etc.) but there is an important distinction. A gentleman sails a boat, not rides a yacht. A gentleman plays golf or tennis, not hangs around a country club. A gentleman host a party, not crashes one. Also, If you can instantly identify the manufacturer of his clothing or if that fact is obsucred by the sweater around his shoulders, he's not a gentleman.
Crunchy: A gentleman is into hunting, fishing, hiking, biking, etc. He might fancy a beard and a thick flannel shirt. But he draws the line at pony tails and Birkenstocks. A gentleman knows how to start a fire, but he's not likely to "burn one", if you catch my drift. If he's wearing a wool cap indoors or no shoes outdoors, he's not a gentleman. Furthermore, he's not likely to enjoy incense, drum circles, or anything else associated with being a hippie.
Slick:(aka Guido or Greaseball) A gentleman likes to dress in a shirt and tie, as long as they didn't come in a box together. In fact most suits off the rack, shiny clothing and gold jewelry are right out. And though he grooms himself well, he refrains from dousing himself in cologne, hair gel and self tanner. See also: faux distressed jeans, pointy sideburns, or crazy printing on a shirt. If you can smell him before you see him, he's not a gentleman.
Shlub: The largest group of non-gentlemen. The world if full of them. They're grown respectable men of serious manner and life experience. In fact, you might easily confuse them with gentleman if it weren't for a few key signs. Jean shorts, a ski jacket (not skiing), or pretty much any article of clothing that is completely unstylish or available at Wal-mart or JC Penny. See also: cell phone belt holsters, sweatpants, fanny packs, random ill-formed baseball caps, etc. If his wife or mother picked out his clothing absent his presence, he's probably not a gentleman.
The idea of a gentleman lies somewhere between these concepts. He might share habits and tastes with these types, but he's nevertheless distinct from them. I'd also like to point out that sharing a few traits with the above types does not neccesarily place you in that category. There is plenty of space between those designations where you'd probably chart. I'd consider myself to be somewhere in that space. Not quite a gentleman yet, but hopefully not one of these either.
Take note that a gentleman may be close in relation to each of these types, but never crosses the threshold. Also, though not pictured in this diagram, each of the types that occupy more than one category also belong to a larger group labeled Douche Bag.