What has been lost with the ages.
Animals Hate Us
Many animals of old were friendly to beastlords (with good reason), while rather unfriendly to most other classes. Alas, this has been lost to us. All animals after the Luclin hate us with a vengeance. I'm surprised our own pets haven't turned on us.
Thick fur, but not thick skin
Beastlord pets are not even remotely usable as offtanks in groups at the high end anymore. Back in the day, ferocious fluffy could valiantly hold off an add with only a few heals by the beastlord. Even in such places as The Umbral Plains this was possible. Alas, fluffy has fallen, and can barely even remain standing when anything beyond a light blue so much as breaths in its general direction.
Pre-slow? What's that?
Back in the day, one of our major raid roles was getting things slowed fast. After that, tanks would get agro off us, and shaman would override our slow when it was safe. Alas, this also has been lost. Warriors can gain agro many times better than they used to be able to, making it safe for shaman to try slowing early now. Add in the fact that shaman's slows now have superior resist modifiers when before we were on equal ground, and this role has all but vanished.
That bright Light! Ow!
As an alternative to our pet offtanking an add (which no longer works), we have Flash of Light! (which.. no longer really works). Correct use of this spell was vital to surviving many bad situations in the lower levels before SoW. Mid-high back in the Luclin Days, Dawnshroud, Maidens Eye, and The Grey were all places where the spell was usable at least in some areas. Even Plane of Fire had spots it could be used. Nowadays, there are fewer wide open safe spots to use it in modern content.
Go chase that kitten!
For years, a semi-frequently used "unofficial" beastlord ability had been using our pet to single pull. Due to the very risky nature of it, it was primarily only used when soloing or if a group absolutely could not find another puller. However, this ability was abruptly removed with the release of TSS, with no replacement provided.
Uniquity? No more.
Back when Paragon was added (originally we didn't have any "real" class AA's), it restored a very significant portion of HP and Mana. Clerics were the only other class with a similar heal. Now every class and their brother has a similar ability, and our modern version, Aura of Spirit (better know as Purrfection) comes out of the bottom of the totem pole.
We're friends forever... right?
Beastlord warders were once summoned as in innate ability (changed due to bugs). Additionally, our pets used to have the unique ability to zone with us. At lower levels this was rather useful when zoning into a dangerous area, though due to issues with how pet was buffed, it became less useful at higher levels. This was all finally "fixed" a while back. However, this is no longer a unique beastlord ability, as all pet classes have it now.