I agree in general with Rahkan:
...it seems more likely that instead of trying to maintain the original class abilities, items, and pecularities of Everquest they would instead just use the historical background and settings of the game as the basis to make a new campaign world in which they would add a bunch of stuff to make it seem more real. It doesn't seem to me that they would include anything like the 1-60 leveling system or the EQ ability point system. Instead character creation would be standard EQ, with standard EQ classes and races for the most part. EQ wizards would be invokers, enchanters have be the same as their DnD counterpart and etc.
Since we don't really know what kind design parameters SSS is using for their conversion, I stuck to the facts about the online game as it is now, and left speculation for follow-up.
I see two main factors driving changes from the online game to the RPG. First off, a bunch of the mechanics in EQ are pure artifacts of online computer gaming that have ZERO value to a pen-and-paper game. Example would be the entirely fixed 8 spell slot mechanic, or the barely measurable impact of ability scores. The other is a presumed desire to maintain some level of compatibility with d20. This is an important point. Going OGL allows them to deviate from d20, but at the risk of losing the D&D-based market. I'm personally expecting a game at least as compatible as CoC is.
If it were up to me...
I would rescale the levels/exp and ability scores/saves to be in line with D&D. However, the key "do not touch" crux of the d20 license is the character level chart, so they may try to avoid infringement issues by changing up the level and exp scale. (Which would suck, IMO.)
I'd take all the various skills, class features, and the "alternate experience" abilities and completely reevaluate them as d20 style skills and feats, with appropriate prereqs and trees. I'd discard the chaff, and avoid re-inventing the wheel, using existing d20 skills and feats where possible.
I'd emphasize some of the cool character class bits that weren't ripped straight from AD&D. The Shadowknight is a very distinct class from the DMG Blackguard. The EQ Shaman is a very cool class that was never even approximated in D&D until the recent 3E & d20 stuff that came out in the last year.
I would maintain the unique identities of each of the spell casting classes, like they are in the online game, rather than make Wizards simply invoker-style specialists, for example.
I would use the d20 combat system, and simply add a feat here or there for EQ flavor. I might be tempted to add an armor location system too, for mix-and-match armor. (Not a hit location system - EQ doesn't have that either.)
I would create a new magic system based on a simplified mana pool. I say simplified, because the bookeeping for mana regen could get quite onerous. For example, Clarity might give you an instant mana dump, rather than an increase in regen rate. Mana points would be scaled down to manageable numbers (like power points in PsiHB). You would want to slow down the mana regeneration considerably for balance reasons. Mana could be regenerated outside of combat by using the Meditate skill/feat, or inside of combat (or any other time) with spells (Clarity, Cannibalize, etc.). The Gift spell line (Gift of Brilliance, etc.) would temporarily increase the size of your mana pool, just as they do in EQ. I would rather see something like this than just more D&D spells. Not that I wouldn't like to see more spells, but you'd lose a lot of the best parts of the EQ magic system by going the standard D&D spellcasting route.
I'd replace the D&D alignment rules with a new Faction system. Not easy to do (elegantly), but the results would really pay off, I think.
Similarly, I'd at least think about taking the simple, generic crafting/magic item rules from the PHB & DMG as a starting point, and then supercharge them with kind of rich details found in EQ.
I'd make sure that the monsters are statted in a way that makes them reasonably portable to D&D. This is a major marketing point for me. There are a bunch of new special monster abilities in EQ that I'd model in a similar fashion to D&D to maintain that compatibility.
I'd do the same with magic items as well. For example, D&D already has magic weapon enchantments that activate on a critical hit - I'd model EQ weapon "procs" the same way.
Voadam, the reason for the spells being available in staged levels is to provide extra motivation/reward for hitting "major" levels, as well as a means of differentiation between classes. If you ignore for the moment levels 51-60 (which were added in an expansion), you'll notice that the divine (WIS) spellcasters generally have to wait longer to get new spells than the arcane (INT) casters do. The classic hybrid classes have to wait even longer for new spells (in fact, they gap between new spells for them widens as you level up). Getting at least one new song every level is a class feature of Bards. The rules change after 50 because the expansion levels are incredibly tough to level through, so everyone typically gets new spells every level. Each new expansion brings more spells too. That spell catalog I linked shows the "era" of each spell based on which expansion the spell was released in. Chronologically, the expansions are Kunark, Velious, and Luclin. The Luclin spells only went in last December, for example. The next expansion (Planes of Power) may be out as early as this October, and you can expect to see more spells added then.