I don't know about anyone else, but for me it's the tedium of construction plus the annoyance of checking the Minecraft wiki all the time that prevented this from grabbing me. The survival/exploration aspects might be bigger hits for me if it weren't for those drawbacks.
True, the UI could certainly use improvement.
Having a way to easily search your stuff and also view the recipes in-game would make it much more pleasant.
The in-game discovery of item recipes is something that's definitely lacking for new players. There's a great mod that fixes this, Not Enough Items (NEI). It adds a nice in-game gui that lists all of the items, what their recipes are and also what recipes they're used in.
The thing that bothered me most was the eternal groveling for resources when I'm trying to build something huge and awesome. Oh boy, my pickaxe broke, time to go spend another hour finding enough iron to make another.
Well, I should clarify, I like some challenge, I just sorta wish either mineral veins were bigger, or they were more common. I'm happy to go digging for diamond, I just wish it didn't take me an hour of random cave-wandering and digging only to find 2 lousy diamond blocks.
You're doing it wrong. Not that I can blame you - unless you take the time to work it out, or find someone else to tell you, it's easy to have trouble finding resources.
The key is knowing where they are, and the best way to get them. Diamonds are only near the bottom of the map, and the best way to find them is to dig long tunnels just above the lava layer. You take a few iron tools and some basic equipment with you, dig a long tunnel until you've broken half of your iron pickaxes, then dig a parallel tunnel back to where you started. You'll pick up enough iron to replace all the tools you broke and then some, a lot of coal, and a reasonable amount of rarer materials.
It's still time consuming, but tends to be much more effective than wandering around caves or digging randomly.
Or you could cheat, and use some mod that helps you find ore. There were mods that add xray vision, or auto maps that can show the position of hidden ores.
Or just use creative mode, if you just want to build stuff.
(And I still like Lego.)