Honestly the main reason I would never convert to 3.5 is because of the spell system. The "designer" who took a knife to it really should not touch 4th edition at all.
There are so many things wrong with the 3.5 spell system compared to 3rd edition it's absurd. It is literally a hack job.
A far from complete list, starting with much of the absurd working my way to just bad changes overall:
*Darkness creates light - it lights up dark rooms
*Geas has weaker effect than lesser geas
*Forbiddance doesn't forbid - it just sorta hurts, somewhat
*Overland flight is the same level as TELEPORT
*Teleport has fixed range - this was the presented "logic": so that people can't teleport to the moon, or similar. Anyone with ANY experience as a DM in D&D: "so they'll just plane-shift then plane shift to within 400 miles then teleport??!?! [/i].
*Shapechange grants Su abilities - anything from infinite vorpal swords (balor) to infinite hit-dice (barghest). I'm sure if you actually knew what you were doing when "designing" things you might work down the list of monsters and look at their Su abilities. Maybe starting with the monsters that start with "A", then moving to those that start with "B"...
*Wish in 3.5 no longer has a cap for items you can wish for. So if you find a ring of three wishes - wish for another ring of 3 wishes... This was debuged in 1E.... Anyhow...
*Meteor swarm is weaker than flamestrike at higher levels. Yep. Because 3.5 resistances
changed to a DR mechanic rather than a round based mechanic. Each burst does piddly fire damage so anything with any fire resistance (read: everything at higher levels) basically takes the bludgeoning damage. If you hit, and with 4 attack rolls only one of which can have a true-strike applied, it's far from guaranteed.
*Polar ray is ridiculously weak for it's level. An 8th level spell that does 1d6/level to one target no-save is well, not quite up to speed with oh I don't know, prismatic wall? Clone? This is Otiluke's freezing sphere ray version, and appropriately level 6.
*Creeping doom is likewise ridiculously weak for it's level in 3.5.
*Blight is quite literally a waste of space.
*Holy word has no cap - it can be broken beyond compare.
*Power word kill is now an "enchantment" - pardon? - and it doesn't affect multiple targets anymore. It shouldn't be 9th level.
*Imprisonment has a save. It is now pointless. A touch spell which can be fixed by 9th level magic and has a save. It's not not even half as good as Destruction which is 2 levels lower.
*Project image is 7th level instead of 6th. Why? Were there droves of people decrying the almighty power of project image? And why can't I cast any damn spells on my damn image anymore? Fantastic.
*Disintegrate is crap. Charm is negated by protection vs. evil. Beholders carry much less punch incidentally.
There's also a definite arcane divine bias:
*Arcane area-of-effect conjurations all become 20 ft radius. But divine area-of-effect conjurations like prayer increased in radius.
*Arcane multi-purpose spells such as Otiluke's freezing sphere, flame arrow get busted into components - but not dispel evil for some unknown reason.
*Arcane spells like Horrid wilting must now "adhere" to a d6 but divine spells like "holy smite" can stay a d8.
*Arcane spells must "adhere" to the damage caps listed in the DMG but we'll tacitly ignore the druid's flamestrike, and make "harm" d20/level - because that's what it is effectively in 3.5.
*The druid "absorbed" a good chunk of the arcane spell list. Rumor has it the "designer's" wife plays druids.
*"Baleful" polymorph, the first true save-or-die spell is 5th level for wizards and conveniantly 5th level for druids and at the same level clerics get slay-living and plane shift.
*Plenty arcane spells rose in level, plenty divine droped in level. No reason really. Dude just felt that druids needed ice-storm at the same level as wizards.
*Conjurations now "mysteriously" have in large part no SR. This leads to most conjurations being significantly better than evocations at doing you know, damage. It's a paradigm shift, and a bad one. Only in 3.5 can a wand of melf's acid arrow kill an iron golem.
Anyhow, I could go on and on and I won't. I'm simply hoping to high hell that 4th edition uses 3rd edition as its basis and not 3.5.