The out-of-office reply is a handy feature that lets you respond to emails automatically with a message saying that you are out of the office, or anything else you want it to say. Just about every email provider offers this feature, though Apple's Mail app does not have the option - you have to set it up on your Mac with Mail rules, and they are client-side, which means your Mac has to be on the whole time you are away in order to respond with your message.
There are many apps that will manage your email accounts besides the iOS Mail app - Thunderbird, Outlook and Gmail will all do the job, to name a few. Outlook is popular with a lot of businesses, so if your work email is managed by Outlook you might want to add your other emails just to be able to access them all in one place. Or maybe you prefer the Outlook interface to that of Mail.
Apple has replaced iPhoto with the new Photos app and launched the iCloud Photo Library feature. Now, if you want, every photo you take with your iPhone or iPad will automatically upload to the iCloud Photo Library. You can even use the library to save the full resolution images while leaving smaller, optimized images on your iOS device to save space.
iCloud.com behaves this way because you should, theoretically, be able to do everything you need to elsewhere on your device. If you find yourself in a situation where you need to get into your iCloud account through the web, you can still do it by tapping the share button and tapping "Request Desktop Site." This will get you there but it won't be pretty as it isn't optimized for mobile.
HomePod owners already know that Siri can help with a variety of tasks. Asking Siri to deliver the weather forecast, play music, or answer iPhone calls is just the beginning. For anyone using Find My iPhone to track a lost or missing device, Siri on HomePod can help with that, too.