Durability, weight capacity, and accessibility are indeed crucial factors when selecting a mountain bike pannier system. However, let's not overlook the importance of weather resistance. Long-distance rides and multi-day trips often expose your gear to the elements, and a good pannier system should be able to withstand rain, mud, and other harsh conditions.
As for attachment systems, rack-mounted panniers are a traditional choice, offering stability and weight capacity. But they can be cumbersome and may interfere with the bike's suspension. Seatpost-mounted systems, on the other hand, are more compact and flexible, but they may sway more during rides and have lower weight capacities. Frame-mounted panniers strike a balance, providing good weight capacity and less sway than seatpost-mounted systems.
In terms of innovations, roll-top closures have improved water resistance and ease of use. Some panniers also feature exterior pockets for quick access to essentials, and others have built-in rain covers for extra protection.
In conclusion, when choosing a mountain bike pannier system, consider durability, weight capacity, accessibility, and weather resistance. The attachment system you choose should depend on your specific needs and the type of cycling you'll be doing. And don't forget to look out for innovative features that can enhance the usability and functionality of your panniers.