The difficulty of a game is a made up of so many factors. To simply say that its easy or hard, is to act as if one facet of the game is all there is, and it only exists in relation to your personal ability.
I'll speak on PvE Raid Difficulty, with the credentials of raiding everything since pre-T1, with the exception of the end of Aq40 & Original Naxx.
Original WoW raiding was harder, but not because things were necessarily more complex.
- Resources & Info were sparse. There was no Bosskillers, EJ, Tankspot, WoWHead, or WoWWiki. There was only Thottbot, which is/was crap. Most people had to figure stuff out on their own, and ended up with "sub-optimal" setups. But that didn't matter, because there were no sites to tell you you were bad, or ways to evaluate your performance (WWS, Recount).
- Classes didn't have nearly as many options. Each class really only had one raid-viable spec. All druids were resto. All mages were fire. All priests were holy. All warlocks were useless (kidding! ...sorta) There were less spells, and less gearing options for them. No gems, and far far fewer enchants.
- Gear mattered. Alot. And a 40 man boss would drop two pieces of gear. You spent alot of time in the blues you got from five mans, and you had to farm the hell out of one tier before your guild was ready for the next.
- Mechanics were simple, but hellishly unforgiving. UBRS in its original form was the big wake up call for most players. Mispull, and you wipe. Let a mob run off, and you wipe. Overaggro, and you die, contributing to a wipe.
People that started playing in Vanilla WoW have a distinct advantage over others today. If they hungered for information then - then they can gorge themselves today. There is more info then can be digested, which helps turn yourself into a finely tuned killing/healing/tanking machine. There are also skills that were learned in Vanilla WoW, which are being used today. Not standing in things, breaking fears, interrupting, dispelling/decursing, positioning, cooldown usage, being "the bomb" - all stuff that if you've been doing for Eight Tiers, your bound to be awesome at by now. There are also the "soft skills" of being prepared for raids, being resilient in the face of wipes and wipes, listening to (or being) the raid leader, and working as a team.
And as far as difficulty goes - tons of content was "inaccessible". You didn't have a snowflakes chance in hell at getting anywhere without being in a massive guild. I never saw the inside of Naxx-60, and only the beginning of AQ40. I sat in IF, looking at the near-full T2 raiders in awe. Being good was special, and not being good was still tolerable - there was always dungeon set 0 to work on (which wasn't too shabby), and the upgraded versions for the truly hardcore five man-er.
Burning Crusade did things different.
- Raids got more complex, and the community responded. Bosskillers reined supreme amongst info, and many a guild got by on their sweet ambrosia of published strats. WoWHead blossomed into prominence. Due to the high availability of info, raids got arguably "easier".
- More class/spec combos became viable. Warriors were no longer the only tanks. Retlols, Lolkittys, Oomkins, became somewhat less lol/oom, but still struggled for acceptance at some levels.
- Playstyle became more important. Gems, more enchants, and more talent options ment that people needed to play well to do well. Well played started to become more important then well geared. A Kara/Badge geared player that knew their stuff could perform decently enough to break into T5/T6 raiding.
- Content became increasingly accessible, as the game went on. Hard content was difficult to access at first. New badge gear made content easier, by letting people get gear from the tier they were attempting (or the one past that) - without needing to kill bosses from it. Attunements were lifted over time, and many bosses had incremental nerfs - leading up to the 30% nerf of everything-raid in 3.0.
- Leveling got nerfed. During BC, Elites got taken out of vanilla wow content, and the leveling speed was increased. The potential for learning about your class from 1-60 was shrunk.
BC-Babies never lived in a world without easy information, or small group (heroic) epics. Blues became the baseline, and class changes based on PvP/Arenas were the norm. Kara introduced many popular/common boss mechanics for new players, that while present, were forgiving. (Void Zones, Kill Adds in an Order, Dragon Breath, Raid-Wide-Fears, Switch-and-Burn, Kiting, Killing multiple things in a time window, Move/Don't move, Repositioning Bosses, ect). 25 Man raiding was still required to get past Kara.
And now Wrath.
- We have info on everything before it comes out.
- Fights are more complex then ever, but more is known about them then ever, and we have boss kill videos coming out from top guilds/sites as soon as the content is accessible.
- Every class/spec is viable, more or less. This is the result of homogenization, and the often abused/misused promise of "bring the player not the class".
- Gear hardly matters. Fully epic players can do 1100 DPS. Full blue players can do 3000 DPS.
- Playstyle is king. You have your choice of whatever class you want to press buttons to do the job you like.
- Content is highly accessible. Different levels of difficulty are created through the use of achievements.
Anyone that can press buttons can raid. From OS+0 10 (3 Manable) to Uld-25 Hard Modes (Uncompleted by Ensidia), there is some difficulty that matches where your at as a player, and something that you can work towards. It may mean that your coming into the raiding scene able to play better then 80% of the content, but until you complete all 100%, there is still stuff to be raided. Its not a bad design - just radically different then how its been done in the past.