The main advantages of a rugged device are easily noticeable in the exterior design – normally with additional rubber-like resin molded on strategic parts of the product's housing. Top manufacturers research and select the most suitable resin for a variety of operational conditions and use cases, then spend small fortunes on dual-injection and other techniques to produce a really sturdy housing. Coupled with a well-thought mechanical design, this is the basis of the structural strength and integrity needed to qualify a product for, say, 10 feet drops or excessive bending, impact, etc. forces on the product structure.
Water-resistance is achievable through gaskets and sealing, and is overall easier to achieve than true structural ruggedness. Nevertheless, consumers should not take a waterproof marking as invitation to use such devices excessively under water. Many devices have gaskets for SIM card slots and other openings, which may not be sealed well after use, which may compromise the waterproofness and allow ingress that can destroy the device. The top brands use waterproof connectors, eg for audio headsets, USB, etc. and do not allow removal of the battery – all additional measures to minimize ingress risks. Even though some manufacturers offer exceptional 3-year comprehensive warranty, it is not wise to misuse a product or intentionally test its limits. For example, if a device is marked IP67, or higher, this means protection against occasional submersion to certain depth and time, it does not mean the device should be used as underwater camera or be washed in a wash machine.
Smartphones use big screens, which are a difficult element to ruggedize and protect. Contrary to some marketing statements, Sapphire glass-lenses are not suitable for rugged phones. They are very hard, which is good for scratch-resistance, but inflexible and prone to cracks. Gorilla-type lenses are flexible and better against impact, side forces, etc.
Another feature, which rugged smartphones normally have, and should have, is a large battery, which allows extended use without recharging. While the regular iPhone can barely last more than a day-use, top of the line rugged smartphones may last a week or longer. This makes a lot of difference when you need a phone out in the wilderness, during days away from ability to recharge, but is also very convenient for 1-2 day trips where you don't even have to bring a charger because you are certain your battery capacity will be enough.
Rugged smartphones also have a louder audio. This does not necessarily mean a higher-performing audio, but certainly a more hearable audio, as it is design to penetrate and be hearable on the background of noisy working conditions. You can certainly play music on rugged smartphones, but they are not designed for multi-media, but to work as walky-talkies, in hands-free conditions, etc. That is also why many such devices have forward-facing loudspeakers and the loudness of these speakers is constrained only by safety regulations and usage patterns.
Altogether, the features of a rugged smartphone result in a heavier and somewhat bulkier device, and in some design and performance trade-offs, which are prioritized on such type of devices. I often see reviews online or in magazines, which compare a rugged device with an iPhone or the like. That is missing the point. The good thing is that more and more people, which are interested in rugged devices, understand the trade-offs and are quite sophisticated in their search.