The most annoying problem I frequently come across is badly implemented authentication for hotels' wifi internet which makes it unusable on a smartphone. You are often given a username and password, each around 8 characters with a validity period of at least 24 hours or the length of your stay. Many hotels have a server-side bug in the their wifi system whereby it unnecessarily deregisters devices if they do not maintain a continuous wifi connection. Smartphones, by design, save battery power by repeatedly disconnecting and reconnecting to wifi networks. Therefore every few minutes, a smartphone has to re-authenticate, requiring the user to tap the screen at least 20 times (8 characters/digits each for the username and password plus navigating around the screen and keyboard). This repeated tedious procedure makes the hotel wifi unusable on a mobile phone. Even worse, if you're waiting for an IP-based communication (e.g. Skype, WhatsApp, Viber, iMessage etc), you often don't receive it because the hotel's wifi system has silently kicked your phone off the network, and nothing alerts you that this has happened. This could all be fixed with a server-side software or configuration change so that each device's MAC address remains registered and authenticated on the server until the end of the username's validity period, but many hotels don't care or don't bother to rectify it. Even better than browser-based authentication, a simple WPA key, as used by bars, restaurants and many smaller hotels, would be much more user-friendly.
Another problem, albeit decreasingly common, is shower heads that are permanently fixed to the wall rather than the more common hand-held design. This is a particular problem in the United States, less so in Europe. How is one supposed to wash one armpits and more intimate areas properly with this ridiculous design? I also wonder how the housekeeping staff can properly wash a shower cubicle or bath when the shower head is permanently fixed to the wall. Shower curtains, as opposed to shower screens, are also very irritating.