Hooray!
This game is really going to shine in speed and challenge runs. After 100%ing the game and doing a few speed runs myself, you really start to realise how genius the level design is and how interesting the gold and item systems are when you are coming up with routes to get through the game as quickly as possible. There's a lot to consider and I think there will be a lot of variety to how people try to run this game. The inventory / gold system means you will have a limited amount of gold to spend and will have to weigh up your options on what are the very best items to spend your time and money on getting. The levels have so many small details and speed tricks on top of this, there really is a lot to sink your teeth into if you are the kind of player who attempts high scores / low times.
The relics are useful in every level of the game, I've seen people who think they don't impact the level design much, but I think you need to look a bit harder. In particular the propeller dagger, phase locket, mobile gear and dust knuckles allow for a lot of interesting shortcuts, ways through enemies/obstacles and quickly killing bosses. The game doesn't require them and they aren't specifically needed to make it through anything aside from the item specific challenge levels, but this just opens up a variety of approaches to every situation. They have made the relics useful based on their utility rather than designing them to be the key to specific locks and I think the game is stronger for it. Speed runners are including the purple armor in their route so they have more magic to use their relics more often.
Here is the current best time on Personal Best Tracker(
http://www.pbtracker.net/game/shovel-knight): 56:50 by Capndrake. If you are curious about how people are already routing this game and how they use gear and relics this should be interesting.