There are so many different Flickr groups and the people are far more like-minded. There are some Flickr groups that specialize further with just Lego decals or castle builds. You can favorite a Flickr group and be guaranteed to see stuff you want. In this sub you mostly get links to box pics, hauls, and set builds. Nothing special at all with those pics. Sure some may be nicely photographed but most have no imagination whatsoever. Boringsville. Same goes for the kind of stuff some parents 5 year old built. Congrats that your kid graduated from chewing on Legos to building with them. You don't see much of that posted in most Flickr groups. Why? Because those using Flickr generally understand the desire for quality pictures, both with the content and with the picture setup. Those groups would fail miserably if everyone posted all those box pics that this sub seems to love. They are a professional photo viewing site like smugmug and others. Imgur is an 'all purpose' bulk photo dumping site meant for quick browsing and downloading. With an all purpose site you generally get all purpose photos. You also have a bit more copyright control with Flickr.
You don't have groups with Imgur. Not like the extent that Flickr has. You have to rely on Reddit groups or other websites as the starting point. You can't really just pop open Imgur and browse hundreds to thousands of Lego pics. With a few groups favorited in Flickr, you can.
Yes, Flickr's site has much to be desired, but I actually like that photos actually take more time to download than right clicking them like in Imgur. You can't just do a mad dash to download an entire users photos.