Per discussions I have participated in over the past two or three years you should be able to watch on a small screen - i.e. a smart phone - and the "sound design" should accommodate tiny speakers and ear buds.
The thinking is that a web series is something you can watch during your commute, or during your lunch break, something to share with friends.
I agree with the eye candy observation - apply it to both sexes; the girls will go for cute guys as well. Also, very obvious quirky characters can be an attraction. Keep it light and fast paced.
I have to say it; sound is even more critical to get right while in production. You won't have the time or resources to do audio fixes so you need to pay close attention to your production sound. Remember the viewing/listening format - small screens & speakers like iPhones, iPads and laptops.
Have the first season of the series completely outlined and two thirds scripted before you begin production. Get three to a half dozen episodes in the can before hitting the web so there's a buffer in the event of an emergency or two.
Streamline the production and the post process, you're putting out a weekly product, you don't have time to screw around. Rely on your talent and crew to expand the concept and allow them a lot of creative input; this is going to be an on-going team effort. You may be writer/director and in charge, but using cast and crew contributions keeps the interest up of all involved. Repeat once again, this is a team effort and requires a slightly different mind-set than dong short or features.