Staying in a hotel with crappy Wi-Fi is frustrating, but not half so frustrating as finding this out by starting up your phone, connecting, realizing there's no Internet access, then manually disabling Wi-Fi to make sure it goes back to 3G or LTE. In Lollipop, Android 5.0 will do that for you: when the system sees no connection to the Internet (or more probably Google servers) via a Wi-Fi connection on a device with a cellular radio, it will automatically default to mobile data to keep an active connection. Neat!
There's also a stronger visual indication of connection problems on both Wi-Fi and mobile data. Previous versions of Android used greyed-out arrows or alternate colors to indicate connection issues, something that a lot of users found to be a less than obvious clue. So now, both Wi-Fi and mobile data icons will add a tiny exclamation point, Metal Gear Solid-style, to indicate no connection or other errors.
Admit it, you can hear the "!" noise now.
All of this is part of the adjusted and improved way that Android 5.0 informs users of the various statuses of the phone or tablet. For a less subtle example, check out the look at me shade of orange that the status bar and navigation bar shift into when power saver mode is activated.
Thanks, Vibhor Mittal!