I need a tent that will work in snow, sand storms, lots of wind, rain and sun(UV). Temperatures I'm expecting are -2C at night to 15C during the day. I will be backpacking, so the tent's weight should be sufficiently small for that.
How do I cut down on weight without giving up what's needed?
I've been told to avoid fiberglass tent poles as they have shattered on others out there.
Are there key attributes mountaineering tents have over other tents that I need in the described situations?
Skirt. If you care a lot about wind and sand, then you should look for a tent with a skirt - many moutaneering-specific tents do have one. But skirt does increase the weight, so there's no right decision.
I'd suppose a tent, that has a very small gap between a... uh, upper part, and a ground. Then you can just put some snow or sand there, to close the gap and have a nice sleep.
As for wind... well, half-barrel-like tents are bad against the wind, so you might want something with crossing poles construction, like half-sphere - but that's not mountaneering-specific, of course.
Personally, I think that most decent backpacking tents will be fine for Iceland.
The temperature is not extremely cold and you should be fine as long as you have a semi-decent sleeping bag.
One thing I would recommend is getting a tent that can be self supporting (i.e. doesn't need pegging out to stand up) this can be very useful if you have to camp on rocky ground, they also tend to be a bit more secure in the wind.
I'm not too experienced with sand storms, but as njzk2 says try and get a tent with a non-mesh inner as this will give you an extra layer of protection. Also have a look at your route to for the risk of sandstorms. A quick search indicates that sandstorms are mainly an issue in spring/autumn and primarily in certain areas to the south of the island so they may be not too much of a concern.
For tents it is generally a balance between weight and sturdiness/protection/warmth. You can get some very light tents but they won't offer you the same protection as a slightly heavier one (at least not without being very expensive). Personally the tent is not the first place I look to make weight savings.
Finally, I wouldn't worry about fibreglass poles. I don't think I've broken any that weren't over 5 years old. Even if you do they generally just snap or splinter more than shatter, and can generally be fixed with some Duct tape and maybe a stick (Duct tape is always essential kit).
