You can never really tell who is hot or not just by their avatar, but once you hear their sweet, sultry voice, you know they have to be hot. Because everyone with a hot voice is hot in RL according to this guy wearing a gothicatz costume, and he was from New Zealand so he would probably know something like that for sure.
Usually I hear girls comment on guys' voices as being sexy when they are deep and groggy-sounding. So maybe this means they look like Vin Diesel in real life?.... I'm gonna go out on a really short limb and argue that he probably doesn't. I've researched this subject for a few years and believe it or not, the only valid assumption one can make about a deep voice is that they dude has probably been sitting in front of his computer (just like you!) for days and hasn't lost the morning grogginess that he woke up with 4 days ago when he first logged on. I can't officially comment for girls, but for me, my voice gets deeper if I've been vegetated in my computer chair for a good length of time, and that kind of lifestyle makes it hard to look sexy in RL. But honestly it's not really a huge deal because gothicatz is always around the corner, whipping up new costumes--and if you're lucky, maybe you will show up to the infohub wearing the newest one first and everyone can admire it for 5 minutes before they go buy it themselves.
I haven't noticed much of a trend for female voices. I have seen sexy voices on RL ugly people and vice versa, more often than not. I will say this (to the guys), don't let yourself be tricked by a super cute voice. Sure you have seen a real life picture, but somehow... some way... I'm almost positive that I've seen that picture on myspace before--or at least it was pretty similar: face drowned out by bright light and camera angle that makes 250lb people look 100lbs.
I'll share a secret with you (you're welcome): When you see a picture of someone with a bright light on their face, it hides any type of blemishes and slims the face--sometimes quite drastically depending on the picture and shadowing. Another thing to watch out for are bright, unrealistic colors. This is a sign of another common way of "quick photoshopping" a picture: boosting constrast/saturation. It makes skies bluer, skin tanner, and pock marks invisible! If you notice any of these things, I'm not saying it is a given that the girl must be ugly, I'm just saying be careful.
Last but not least, I would like to officially announce that people who eat and slurp their drinks on mic are disgusting. No, I'm not "giving you a hard time," I am dead serious. I think this is something that you should be able to report and result in a permanent ban. Maybe that's a little extreme... or maybe hearing you smack that macaroni and choke that corndog down your throat in HD sound quality is more repulsive to me than seeing a naked avatar in a PG sim. Do everyone a favor and turn off your mic. NOTHING is so important to talk about on SL that you can't take a 5-minute break to eat or slurp your coffee. Of course this is just my opinion, but I think this should officially be made into a reportable offense.