There are various monsters in LOTRO that make the loremaster in me scream. The Nameless in Moria are one example. They look like aliens--and not just any aliens, but the aliens sometimes called Xenomorphs from the Alien series of movies. Tolkien wrote of nameless things, yes, but not aliens! The ones like World-eater in the Rift of Nurz Ghashu and the Void-eater from Dark Delvings also qualify.
And then don't get me started on the Gaunt-men and their lords. If these gaunt lords were so powerful and strange, then why were they never recorded in elvish history? Surely if they had a basis in lore, they'd be fine, but no work of Tolkien's contains any of these things.
Same with those Caerogs...what are they anyway? Earth-elemental Balrogs? Escapees from my D&D game?
And let's not forget Gwathnor, that interdimensional monstrosity you fight in the instance "New Devilry". Just look at him! He's practically a World of Warcraft refugee! He does NOT belong in a Tolkien based game! Even bloggers have hated on this monstrosity!
Finally, more natural creatures also offend. The Gertheryg are supposed to be sun-resistant, intelligent trolls from the First Age. Where do they come from, and more importantly, why the hell do they even exist?
Same goes for Fire-orcs, Fire-giants, and various other lava-skinned perversions. Those guys are just plain bizarre.
Of course, we have things like the Fell Spirts, Ancient Evils, and various other corrupted things (such as Cuthraul and Druggavar). Those have a basis in lore, as the spawn of Morgoth after being cast into the Void.
But what are these other disgusting things doing in my Arda? How the hell are these Tolkien monsters at all? I get that you need variety and we can't have you fighting orcs and trolls every day, but I can't believe that nobody has brought up the brutal murder of lore these things commit!