Even though I don't think many people consider board games to have user interfaces, technically they do. It's the components themselves. The parts are the things that the connects the players to the rules. When games are considered to be "too fiddly," that's just another way of saying that there's too much stuff on the interface. However, whether that's because there's too much needless stuff, or too many rules, is another issue, one that's primarily left up to a player's taste, for the most part.
I'm pretty amazed at some of the awful interfaces people have designed for various uses. I've seen gas pumps with a START button nowhere near the rest of the button panel, for example. Not very intuitive. There's quite a few websites that deal with bad/good interfaces which are fairly interesting to read.
In boardgame design, however, I think that most interface issues seem to fall into place fairly quickly during playtesting. You see very few games with insanely stupid interface decisions. You are more likely to see some bad interface designs brought on buy cost considerations more than true design thinking (or non-thinking). Such as using non-language dependant symbology for cards. It might look cooler, but having readable text that you understand sure helps a lot; however, if you can bring your cost down by only having to do one print run for the whole world of components, so much the better.
Anyway, where all this is going, is this: Someone emailed me about PocketCiv (which is becoming a lot less pocket-ful as I go, due to the work I'm putting in on the deluxe version), and wondered about the counters I had made for the City Advances (the red and blule counters in here). Basically, the question amounted to why i decided that each counter is a +1 AV counter that needs to be stacked, as opposed to simply having counters that get swapped out that said 1AV, 2AV, 3AV and 4AV.
I had considered this at one point. But when playtesting, I found that I sort of enjoyed the stacking aspects when a city gets built up. Aside from being visually pleasing, I didn't need to have to go searching for a specific tile with the right number on it, just grab another AV chit and stick it on top. And if an AV got reduced, simply pull one off the top and throw it in the pile.
Swapping of chits os sort of messy, since you have to search for the right chit that you need, and then remove the old one, and place the new one i the right place. The old chit being removed would presumably have to be placed in an area that you would have to remember where it was in case you needed to swap them out again. Not that this is REALLY a brain-bursting or Operation! twitchy physical action or anything.
In addition, anytime a player changes the state of a board, you are always prone to the player somehow accidentally whacking something that was not meant to be moved. Swapping tiles seems to complicate these issues a fraction more, since you are moving things at the board level. Stacking and unstacking tiles are slightly above board level, and is only one action, reducing possible whacking time.
Of course, there's always the condition that you now have stacked stuff that can fall down while you are moving other pieces around. So it's not THAT foolproof. But, hey, you go with your gut.