What I look in a performer is how intensely into the music they are, visually and physically. And the biggest one, IMO, is whether they are engaged with each other. Nothing is better than seeing a bunch of guys who are constantly playing off each other, feeding off their energy, making eye contact, and feeling the rush of each-other's playing.
I like to tell people that I play live like I'm driving a car: I focus on the road (audience), the wheel (my instrument), but then circulate between looking to the side and rear views every so often (my mates). If we all do that, and connect at certain times, it makes for a great rush that we can feed off of, and the audience loves it too. Don't concern yourself too much with making contact with the audience—that's largely the lead's responsibility, although there's nothing wrong with it either—the most important thing that moves the audience is to see and feel that you're into it and connecting with each-other on stage. As long as the lead vocalist or someone (lead guitarist) is giving the audience direct attention at most times, then they can sort of act as a "lens" for the rest of the group's energy.
Don't be afraid of moving too much. Yes, they teach you it can be distracting (mostly for concert music) but really 90% of musicians have trouble being too wooden than too energetic. Rock 'n Roll is dance music, so it's natural to groove a bit. Keep the beat going with your body... don't be afraid to LOSE IT a little during a big punk or metal number. And I *ALWAYS* stand, I can't even imagine doing what we do sitting down. That's why Jerry and Little Richard could only stay in their seats for so long!
NOTE: I did play in a metal band for a while who felt like it was their job to stand there like a pole and look pissed off. WE SUCKED LIVE. Don't do it. I don't care if you're the most emo/goth group in the world, if you don't look like you're engaged, the audience won't be either.
Finally: Schtick. It works. I have a keytar that I break out for easy, cheesy 80s stuff. It's wireless and just mirrors my main rig, so I can grab it any time it feels right. I have an accordion for Irish Punk numbers, and a $150 plastic trumpet that I use for Motown, Cake, and whatever. The bass player also plays some keys, so we actually switch-off mid-song (American Girl), and then I play bass for a few tunes. Over time we've worked out some haphazard choreography and skits. I get into a fake fight with the drummer during Jessie's Girl. Nice thing about playing keys in rock is that it's a color instrument, and not 100% necessary all the time, so if you have some other instrument that you play a little of, that you'd be otherwise playing on samples, consider adding that in to your bag of tricks. It's a great visual gag that plays well with the audience and adds some dynamics to the performance. Play a little Recorder? Stairway to Heaven. Harmonica? Train in Vain (Clash). Violin? Baba O'Riley or Come on Eieleen. Sax/Brass: Motown, oldies, etc. It doesn't even need to be the original instrument. I grab the trumpet for Careless Whisper and many synth leads. Plus it's fun for me, and when I'm having fun, some of that will rub off on the audience.
As the evening gets really underway, don't be afraid to get a little crazy. Play something easy that you barely know, jump off stage and play in the audience, get really physical with the music. Live a bit on the edge... and even if it doesn't work out, you and the audience can have a big laugh.
