Thanks for the good answers.
I can offer you some finer details. (these are just quickly off the top of my head)
* AC was hardcapped until PoP, so it's softcapped on TLPs. The overcap returns are also more forgiving than PoP era returns. Certain loot like Vulak and AoW loot was given ridiculous amounts of AC with the hardcap in mind, so these items are now super powerful.
* Player combat skill caps were increased significantly. (1h slash, offense, defense, etc)
* Sometime around Omens, Sony significantly increased the damage of level 1-50 spells.
* Monk fist damage is of course much higher now.
* FD is ez-mode now with 2 minute mem wipes.
* TLPs of course have instancing, which means every guild gets buckets and buckets of raid loot with no contesting. Mobs don't even respawn inside the instances, so the challenge of having to deal with respawns is gone. (e.g. protectors of growth in growthplane)
* Many non-instanced raid boss respawn timers were cut in half.
* I don't know if they fixed it yet, or even want to, but warders weren't even working in the VT instance.
* The newer clients allow you to see underwater very far away, making zones like PoWater easier and ruining the atmosphere.
* Some zone NPCs have easier mob stats. (e.g. some Kael trash have lower hp, hit less)
* PoP NPCs expansion-wide ALL hit around 5% less damage for whatever stupid reason.
* New tradeskill interface makes raising trades ez-mode. New recipes with overpowered items remain. (I think)
* The resist curve was reworked just before PoP's launch, making pre-PoP content a bit easier.
* The NPC 'forget' distance was globally lowered to 600 units, which means NPCs will ignore you if you run 3x spell range from them.
* Pets were granted a bunch of mitigation and ATK.
* Focus effects on items pre-Luclin. (TAKP has this and a couple other things I list here)
* Rampage and Flurry are a bit less threatening than they used to be.
* Zone revamps nobody likes. Half the old classic zones are gone. The new Bazaar is the most hideous monstrosity I've ever seen.
I believe TLPs disabled the free Veteran's Enhancement AAs and the Innate Felstrike bullshit. I just want to mention these because they are Fing retarded and trivialize the normal servers. (well, Mercs do more so)
Of course there is also that store, which allows you to buy exp potions, which kills the level race.