Obviously they could add it - it wouldn't be hard. Just because it doesn't have feature X doesn't mean it's bad. At that point it's a conscious decision to exclude that feature. Poor performing mobile sites? That is something they could for sure address, especially with the engineering power they have, I agree.
Twitter's been around for 10 years—and maybe it's just my perception—but I feel like the tech. media has never done anything but hate on them. It's well-founded criticism at times, sure, but at what point does it become a loop of negative attitudes simply based on pre-existing negative attitudes? That's just my take on the whole thing. I personally have some confidence in Twitter and have nothing poor to say about their product / direction.
I took the CS OS grad. course at Purdue last semester that taught OS principles using XINU. Overall, I think the OS is great. Used primarily for embedded systems (routers were the common example) it was simple, quite extensible and fun to learn.
It will change the artwork daily by default, but you can change the settings to change on each new tab.