And I'm saying- as someone who HAS read all the books- that to me, 4Ed is a more radical departure from what went before in a way that 3Ed/3.5Ed were not. What mechanics were excised or radically altered inform my position as much as those things that were shifted from optional to core.
A similar thing, but much more drastic happened between 3e and AD&D.
In fact, 3e had an entire shift of how it worked and is basically a completely different game.
The biggest shift in thought was the way multiclassing worked without penalty, work, or sacrifice. It became more like dual-classing, but without having to meet any pre-reqs overall and not having to abandon your old class (except in rare instances), nor with any limits.
This made 3e more a game of skill packages rather than the typical archtypal classes.
In many ways it was like other games with skills or soft classes (classes with bonuses, but not hard set like AD&D or D&D pre-3e).
This shift towards rolemaster isn't unusual considering some of the people that worked on 3e...and in fact had been incorporating some of these changes as "Optional" previously. Of course, many other drastic changes such as mages being able to continue casting when hit, unlimited levels and class options for races, or even the magic store can be seen by others as changes so drastic as to create an entire new landscape for 3e.
Overall, 3e was incompatible with AD&D or the old D&D without a LOT of wrangling...in fact I'd say Palladium was more compatible than 3e...and they consider that a completely different system.
This wasn't necessarily good or bad, but as for compatibility...it broke most sacred cows and appealed more towards those who didn't like AD&D, but preferred other systems. It showed and brought back those people in DROVES!!!! In fact, for hits, I'd call it a homerun. Backwards compatibility on the otherhand, not unless you REALLY stretched it.
With 4e it is actually quite compatible with 3e. In fact I came out with backwards compatibility guides as well as how to play 4e as a 3e game...got a slap on the wrist (or perhaps in the face, however you see it) and statements as, if they still wanted to play 3e...they'll play 3e.
All 4e really is, is adding in these special "powers" for everyone. Take away the fighter's "powers" (and yes, then he quickly becomes underpowered related to everyone else), along with the rogues "powers", keep the strict restrictions on who can or can't have theivery (though warlocks probably would become immensely popular), and you suddenly have a very 3e like game.
Skills are simpler, Saves are simpler (or if you want, instead of using them as defenses, still roll to hit and then use their bonuses as saves instead). You have a different to hit progression...more akin to the epic rules for BAB increases along with saves...but then...that was already in 3e to begin with.
The BIGGEST and most incompatible change with 4e are how HP are handled. Want to run 3e modules with 4e...simply reduce HP to being what a character gets per level+CONS mod. That'll actually keep most of them under 10 HP for 1st level, but well over the low HP of older editions.
All 4e is, is basically an OGL form of 3.5 with powers tossed in and a few tweaks, far more tweaks than 3e had from the BASE/Core AD&D. The other big problem people have is with Wizards and how to rework from a Vancian point of view to a Ritual/Spell point of view. Rituals are actually REALLY easy for conversion processes...which leaves the spell portion...and that I can agree with you. It's about as hard to do as the spells from AD&D to 3e where a Wizard suddenly gets a whole bunch more spells to cast at 1st level, and many of them are VASTLY different then their older counterparts.