The Lionhead has finally gained recognition by the ARBA, which means there now is an official standard for them. According to that standard, the Lionhead weighs no more than 3 3/4 lbs, and should be posed sitting up (kind of like a dog sits). The standard also has a lot to say about how much wool a Lionhead has and where it is found on the rabbit's body.
The problem with Lionheads, is that they have become ridiculously popular in a relatively short time, with everybody and their best friend trying to jump onto the Lionhead bandwagon without even knowing what a "good" Lionhead looks like. This is a good site to learn about Lionheads:
http://www.lionhead.us/
There have only been a handful of Lionheads imported into this country. With such a small gene pool to start with, outcrosses were essential to produce enough animals to meet the demand. Some outcrosses improved things like wool or type, and some just made more rabbits that had manes.
Another problem is that the gene causing the Lionhead coat is dominant, so any rabbit inheriting one copy of the gene will have longer hair in some areas on the head and neck, and perhaps elsewhere. Rabbits that have one copy of that gene, and one of the normal coat gene have what is called a single mane, which frequently molts out to just a few wisps of longer hair as the rabbit matures. Crossing two rabbits with single manes can result in animals that technically are Lionheads because that's what the parents were, but which have no manes and won't produce maned offspring (unless bred to a rabbit with a mane). For a while, there was a possibility that the official Lionhead might even have been a Lion Lop. Hopefully, now that there is an official standard, some of the excesses (I've seen "Lionheads" that looked like New Zealands with a wisp of longer hair between the ears) will become less common.
One of the "dirty little secrets" about Lionheads is epilepsy. I don't know how common it is in the Lionhead population as a whole, but since a lot of the rabbits that have it only freeze up for a moment or two during a seizure, it's hard to track.