Cumulative UV exposure would be really handy but local temperature not as much since when you're inside it wouldn't be very useful and being so close to your skin, the temperature wouldn't be very accurate as there is usually a warm layer around your body. I used to have a watch with a temperature sensor and it was never accurate because of this reason. Perhaps they could improve upon it today but I doubt it.
As for it being a silly complaint, if you read what I wrote I was more getting at the point that a lot of people try to make as to why Apple won't allow third party watch face development: Developers would design cluttered interfaces that would mar Apple's "beautiful design". This is evidence to the contrary and anytime that argument is used we can point to this cluttered face as an example of why that argument doesn't hold much water. I'd love to see third party watch faces, and I did say that it's nice to have an option. I recognize that we don't have to use this face as there are many available, but saying that it kinda flies in the face of Apple's usual clean design aesthetic. I might use this face from time to time, but not likely as a daily driver unless I can tone it down a few notches. However, I am hoping that complications with the little meter thing underneath are going to be available from other faces as well.
"a lot of people try to make as to why Apple won't allow third party watch face development"
A lot of people may say this, doesn't mean it's true.
Much more likely, I suspect, is Apple concerns over power (and perhaps also memory). Giving an app access to the watch face is giving it a huge opportunity to burn energy constantly, and we KNOW that developers abuse that sort of power, and that Apple then gets blamed. (Consider, eg, how two high profile apps --- Skype, and Facebook --- both burned large amounts of battery, which made many iPhones, and Apple, look bad. And that's from companies that are large enough that they should know what they are doing.)
Every time developers screw up, Apple tightens the screws and tries to ensure that their next round of products aren't as susceptible to that particular abuse.
Temperature, I think, can be handled well through a bolometer. Many "sports" watches (diving, climbing, running) feature temperature sensors, and given what they cost and who uses them, I assume those sensors do their job well.