Simple is not the same as easy. The EQ combat is simple on paper because if you have a group, each class only has to cast a few spells in each fight, but it isn't really easy because everything is tuned to be so on a knife edge. Examples of EQ's combat itself being tough is starting at level 1 as a caster. You only have 2 spells, an offensive spell and a buff which you cast already. You find a single mob so run to near it and cast your 1 spell on it, something like Sicken for an Enc or a little nuke for a Wiz etc. It does about 15% damage to the mob and the mob runs up to you and starts hitting you hard. If you start casting again and the mob hits you during the long cast time, it will definitely interrupt the spell and you wasted all that time doing no damage because a resist is 0 damage from magic but also 0 damage from melee because you don't hit while casting. If you keep doing this, you will definitely die, and many newbies do die like this. You have to auto attack because you don't have enough spell damage to kill a mob, so you need to land a few of your little 6 damage rusty dagger hits to the mob as well as landing some spells to actually win the fight. But spamming your spell will likely mean it gets interrupted 95% of the time and you can die because of that. But if you wait for the mob to hit you and then instantly cast the spell, it will cast while the mobs attack is on a delay and your spell will land every time.
Compare that to WoW, and this is not an exaggeration, in WoW you run up to a mob, same level player, same level mob, and you click the mob once and then just go AFK and watch it. With no spell casts, the character will beat down the mob and the mob will do very little damage back to you, and you also regen during the fight. The character wins the fight with zero input from the player, and you end the fight at 95% health and 100% mana. Compare that to EQ where you had to time attacks with great accuracy, you had to auto attack and you used close to 100% of your mana, and now you need to pray some other mob doesn't join the fight (which is likely to due to social aggro).
That's a level 1 encounter in EQ but going through the levels, this doesn't change that much. At level 30 something, many classes can not solo a mob by just running up to it and attacking and casting spells. The mobs just hit too hard and fast and your damage output is too slow to race it to death. You HAVE to do things like kiting to win. In a group against blue cons it is a lot easier, but white cons and yellows and especially reds can still destroy a group easily. A Kor Wizard or similar in LGuk can almost kill any tank with one spell, and if you are already tanking one mob and then a wizard shows up and starts nuking you, you could die before you even get a chance to run at the wiz. Even a Cleric at this point can't heal the tank to full with one cast, so they must cast multiple heals to recover from that one nuke, and the casts take a long time and there is chance the wiz will nuke the tank again in that time if you are not really fast. Any melee character in the game cannot kill that wiz fast enough to stop it casting a few nukes, it takes a concerted effort from some melees and some casters in the group nuking hard to kill the wiz mob before it kills the tank. At level 50 in Sebilis, this gets even harder. That zone is full of Wizard mobs and they cast Ice Comet which does around 1000 damage and many characters at that point only have a similar number of HP. So if a mob nukes a caster in your group, it will usually take them so low that any other mobs will instantly aggro on them and kill them in a couple of hits. And almost all mobs in the zone come in groups of 3+ but often you get more unless you go really slowly and have a dedicated CC on the ball.
That said, I think it was too simple and too easy if people chose to play it a certain way. It was an entirely different experience to do Lower Guk at level 30-40 ish, than it was to do it at level 49 when your wiz had his new Ice Comet spell. That spell can almost kill mobs in the zone with one cast. Get him in a group with a bard or enc for mana regen and a group can kill half the zone like a killing machine. But that's not how the game was really designed to be played. It's also completely different to go to Seb in early level 50's, than it was to go to Sol B at 50 ish, yet you are still fighting dark blues in both places. EQ was extremely tough if you pushed it and went to Seb, HS, etc, at level 50 ish, even with a full set of old world raid gear, you would easily get slaughtered in Seb. People who say it wasn't tough are thinking of going to Seb and fighting LDC's at level 52 and basically doing content in a way that the game wasn't designed. Necros used to go to Skyfire and pull single mobs to the zoneline and cast snare, fear, send pet and maybe cast a dot or two. Playing like that was easy. The downside is you only got about 0.4% exp per kill so you would have to spend stupid amounts of time there. It was also easy to get a full group and do Karnor at level 57+ when everything is either blue or light blue. It could be easy but it could be really hard too and I think those examples of it being easy are not really how the game was designed to be played. Also however you play it, there is no other MMO that comes close to being that tough.