Hydration bladders come in many different sizes and shapes. For the most part, bladders are available in 1.5, 2, and 3-liter sizes and will either have a circular screw opening or a dry bag-style fold-over closure at the top.
There are pros and cons to both, but look for a bladder that has a big opening – this will make refills, cleaning, and drying easier. A top tip when filling the bladder is to suck all the air out once you've sealed it as this reduces volume and the sloshing effect of the water when riding.
Like bladders, not all bite valves are created equal. Most are made of soft rubber, just requiring a squeeze to start the stream of water, though some allow fluid to flow more freely than others. The tubing that comes with most hydration packs can be cut to length and should feature some sort of a lock-off valve that prevents leaks.
Lastly, look at how the hose is routed and stored. The last thing you want is a loose hose flailing around while you're trying to ride. Almost all bags route the hose over the shoulder and then attach it to one of the straps with either a loop or magnet. Many brands offer the flexibility to choose which shoulder and where it attaches, so you can choose your preferred position.