I like WAC a lot. I like the wide variety of creatures it adds to the game but mostly i absolutely adore the gear. WAC's Mage robes and staffs are my favorite modded robes and staves from any mod. And, of course, Waalx's renowned Real Swords mods (my favorite weapon mods) are incorporated as well.
It is a fairly modular mod. There are 14 .esps (not counting the main .esp and .esm) that can alter how the mod works. One, WAC's version of Crowded Roads, is my favorite Crowded Road variation. It adds backpacks and baskets and some of its other new gear to the travelers, which always brings a smile to my face. I also like WAC's updated version of Natural Habitat. Seagulls flying around the waterfront!
There are some really neat creatures in this mod, like Gargoyles, who are stone until you get too near to them.When you approach them they burst into life and begin flying around an attack you. There are some cool new mounts, like Deer. My Druid-type characters often ride one of these. Some creatures can be tamed with offers of food.
All of the arena changes are located in a separate .esp too. I fixed the gate issue in my game and tried it out, but I didn't really care for it. I love the idea in theory - three teams battling each other and you are no longer the center of attention - but it was just a bit too hectic for my taste. I'm not much of a twitchy, adrenaline player. I prefer combat to be a little slower. I also don't like large battles very much. But I can see how someone else might love it. The mod makes some very interesting changes to the Bloodworks that I liked, though.
About the other problems you mentioned: there is a fix for the Gnolls problem floating around. I know nothing about a Deadra cuirass problem. But I don't always use all of the .esps in one game so I may simply have missed it.
I did have to tweak the mod to suit me. But this doesn't mean much: I tweak every mod I use. I am an inveterate, compulsive mod-tweaker. Some of the creatures' speeds were set way too high for my taste. I slowed many of them (such as the smaller lizards found near lakes and rivers) down quite a bit. I think really fast speeds on anything, player character included, just do not look good in Oblivion. I also had to tone down the followers the mod adds to Highwaymen. I love the idea, but I thought they were a little too difficult.
I'm not an expert on the mod, by any means. I've played only two games with it - totaling about 100 hours - so there are many things I'm sure I have not seen. But this might be enough to get you started.