Not Everyone Is Beautiful
The base line assumption for most PCs is that they're attractive. This is fairly natural when you think about it. The PCs are characters out of a story. They are fit, capable, and confident; all three of which are features many people find attractive. Does that mean they are? Have you ever tried saying your character wasn't attractive and seeing how people react? Some groups are good for it, sure, but in a lot of groups it works like playing Femshep (of Mass Effect fame) when you've deliberately set the sliders to make some monstrosity: everyone is talking about how attractive and pretty you are, when in truth you look like someone an acid drenched demon wouldn't ask out.
Cut A Dramatic Figure
When you get down to it, this is something most of us want our PCs to do. We want them to be the silhouette of sheer bad-assery that makes the bad guys quake with fear. Maybe we do this the stock route of capes and trench coats, maybe we go a bit more distinct and flavorful for ourselves. Either way, who doesn't want to look bad ass? Of course, this only works if people pay attention and thus can get the same image you have.
This is a fun one to test out how much people pay attention in a game. If your character has secrets, hide one of them with the appearance and see if anyone catches on. I did this in one game where my PC was a woman disguised as a man in an all male group. Every morning, and I mean every morning the following exchange would happen:
Me: I'm going to wash up in the river while they pack up camp.
Someone Else: Good idea, I'll do that too.
Me: Nevermind then, I'll go do
Now, this in and of itself could be innocent enough, except it happened every time when bathing would involve being in public. If players were paying attention to how things looked - the character always dressed in "form hiding" gear - you could at least get the impression that there was something on my character's person that I didn't want seen. This is a bit different from the core of this post, but still related.
You can use description to give cultural relevance in some situations. For example, characters with di-chromia (different colored eyes) are seen as being touched by evil in Rokugan. While they're not outright shunned, superstitious people are a bit more nervous about them. Similar things can be done with scars in certain locations - the beautiful duelist with a scar on one cheek for example, or the bad ass with the "I'm a bad ass" scar over one eye that doesn't impede vision for another - or tattoos, or almost any other marking.
One of my favorite disadvantages in some older games was the 'marked' disadvantage. It simply meant that you had some outstanding physical feature that made you easy to identify. A visible and unique tattoo on the neck/face where it was hard to hide, different colored eyes, unnatural colored hair, something. The downside to this was that it made it harder for you to hide. If the police, or anyone, was looking for you they had an easy way to know they had the right person. It also meant that they could look specifically for you and it was harder to throw off a fake trail. Fun times.
The point of all this ultimately boils down to being distinctive. Visual references can be glossed over in table top games because, let's face it, as much as you want to imagine that the rogue is a beautiful and buxom yet slender red head that could rival a super model in appearances, you're looking at your good friend Mark - or maybe his girlfriend Mary. What you are actually seeing can over-ride what you are imagining, and the image gets lost.
However, if you paint the picture right, and you can get everyone to keep it in mind, you can get a lot of powerful mental images with some very basic things. Description for action can mean more as a properly conveyed character image helps put the pieces in order. It doesn't necessarily strike every session, or even every other session, but when it does it leaves you with a moment that is truly memorable. Ultimately, isn't that the whole point?