I'm going to start with the straight answer that I offer students, if that's what you are looking for:
Wear something you feel comfortable moving in, and can see your movements in. Leggings or yoga pants with a vest or t shirt is fine, you can wear a shimmy belt if you like (I lend them out) bare feet or soft dance shoes are good.
But there's a little more to it than that - because otherwise I wouldn't have started writing about it.
I'm not going to talk about footwear though, because I've done it before.
Dressing for class
What dancers wear to class depends upon a number of factors. The style of the class and the preference of the teacher (and other students) dictates much, the dancer's approach to dance and what they are getting out of it, what the dancer feels comfortable and functional in and what stage of their dance career they are at.
Vashti's short article on Shira's site is spot on. I have been through all these stages and I'm now at the point where I have to remind myself when I am teaching not to dress as I do when I am learning, because people tend not to recognise me as the teacher and think I am a scruffy stray who wandered into the wrong room! [I think I am going to have to write a whole other article about dressing as a teacher, because that's a whole new can of worms.]
Skirt or pants?
Some dancers like to come to class in a skirt, the right shape skirt can help emphasize hip movements and if you want to dress up a little for class a pretty skirt is a good option. I often wear a 25 yard skirt for ATS classes (wrapped and tucked) and occasionally a straight or basic fishtail skirt for Orientale classes too. The downside with skirts is that if the class involves floorwork, yoga or certain types of conditioning you might find a skirt impractical. A skirt also covers your legs and often your feet, which means your teacher can't keep an eye on your footwork.
Pants could be yoga/jazz pants, harem pants, leggings or specialist bellydance pants like Melodias. I find longer pants trip me up a bit, so I tend to go for calf length, leggings or harems with elasticated hems. Really full pantaloons can be a trip hazard, but they are OK for styles that don't involve too much fancy footwork.
Fitted/stretchy tops that allow the line of your body to be visible are really helpful when you are learning bellydance. Often your movements will be small to start with and they'll get lost under a voluminous t shirt or layers. A cropped layer for warmth over a normal vest top is great. You don't need to bare your midriff, in fact I don't think any of my current students do.
For the ladies, I would also recommend a sports bra. Especially for saiidi and suchlike. You might get away without but holding back for "lack of support" takes some of the fun out of your class. And while we are on it, for the chaps, Google "dance belt". You are welcome.
Noisy coin belts are sometimes a bone of contention. I encourage them in my classes, because my classes are small and I think the weight and auditory feedback is really useful for learners. In a large class or an echoey venue a lot of students with noisy belts and ants in their pants can be massively distracting (see Stork's sweary guide to sins in large workshops, if you are comfortable with sweary ranting). Also be aware that cheaper coin belts with stamped coins will "shed" as the sharp edges work through the thread. If this happens be sure to pick up your beads and coins before they cause injury.
Hipscarves and belts also serve the important purpose of making your hip movements clearly visible. So go for a colour that stands out. If you don't like the noise you can get quieter beaded ones, or just a bright fabric scarf. I have a habit of using my neckscarf (because we are back to the bit where I dress for class like I had to buy my clothes at a charity shop 5 minutes ago).
Remember that what you are wearing helps your teacher to see your dancing, so the clearer your attire is, the better feedback you will get.
Dressing for workshops
OK, there are probably 2 kinds of workshops, and they aren't always that clearly delineated, it's more of a sliding scale. I'm going to call them "fun" and "serious". Not because the fun ones aren't serious or the serious ones not fun, but for want of a better term.
"Fun" workshops are the ones aimed at dancers who dance "just" for fun. Often they happen before a local hafla or as part of an end of term celebration. They have descriptors like "try out a fun folk dance", "suitable for all levels" and tend to be a great laugh with a bit of a party atmosphere.
"Serious" workshops have descriptors that say things like "bring a yoga mat and water", they are aimed at dancers who want to be challenged and push their dance to the highest level.
There are instances where a workshop will have a clear mixture of "fun" and "serious" participants, and that's just fine too.
For a lower impact, party atmosphere class, you can dress up a bit if you want to. Go for the skirts and your fancy tops, add a little bling (but make sure there's nothing sharp or spiky or generally health and safety bothersome). Don't wear your performance kit, you'll likely still sweat in it, crumple it and generally shorten its lifespan.
Don't wear anything that is going to affect the learning of others in the room, like enormous headdresses (yes, I've seen that) or extremely noisy costuming - shimmy belts are usually still fine although some teachers will ask you to remove them. I usually go for a quiet belt if I don't know the teacher.
If you don't want to dress up, that's also fine. You can join me in the lost property bin club.
If you know your workshop teacher has a thing for yoga/pilates. If they have asked you to bring a yoga mat. If the write up suggests it will be sweaty/physically challenging. If the focus is drilling or advanced technique. Then you need to dress for a workout/general dance class, not a bellydance party.
You are wasting your time and money if you can't take part in the class because you are hampered by your outfit. Avoid long/loose pants you will trip over. Make sure you can get down on the floor, sit in a straddle etc without being restricted or flashing everyone.
Go for leggings/pants and a vest, with layers, which you will need for warming up and cooling down, and a light, quiet hipscarf.
If there is floorwork you are going to need kneepads, if there is spinning/turns you will likely need shoes or dancepaws.
So there you go, question well and truly answered to death. I hope that takes some of the mystery/anxiety when preparing for a dance learning environment, and if in doubt, just keep it simple and functional, you will develop your own style and preferences as you go. .