I've been playing / DMing awhile as well. There are substantial differences between the original AD&D and the new ver. 3.5. I'm currently playing in a group that uses the 3.5 rules, and the GM and his best friend are experts on the new rules. (They really are; that is not a jab at anyone.)
The original AD&D had a certain fantasy "flavor" to the game. A lot of the spells had combat applications, but many of the spells could also be used for non-combat situations. Most of that has been "wrung out" of the new rules, e.g., spells that had hours of duration in the new rules last minutes or less in the new rules. Ver. 3.5 is really designed to be a fantasy wargame as opposed to a fantasy role-playing game with wargaming overtones like the orginal AD&D was.
Here are some specific changes.
Higher armor class is now better. You roll a D20, add your modifiers and try to beat the opponents AC. Not a big deal and makes the math easier on the youngsters.
WoTC / TSR / Hasbro has reduced the number of saving throws from five to three; that is easy enough to keep track of.
They have added a whole new set of abilities and functions captured under "feats" and "skills." Feats include things of vastly varying game application from magic resistance to increased dexterity to dodges while skills are essentially non-combat applications. There are some fairly non-intuitive combinations of feats, etc. that are hard to anticipate or keep track of. (More than half of the time in the last two sessions of the current ver. 3.5 group that I'm in have involved "rules discussions," and it has not been rules "lawyering." These were legitimate questions about "what the heck do the rules mean in this context?" The rules interpretations did not involve my character's actions, but I appreciated why the discussions took some time.) In the old, ver. 1, AD&D, you could have just gone on with a quick table interpretation (the old rules always needed some interpretation), but there are so many "moving parts" in the new system that you have to "nail down" issues or the whole thing collapses under its own complexity.
They have some very weird things now too. For example, you no longer have levels "drained" by undead; now, you "gain" negative levels.
What is the difference? In game terms it adds a layer of complexity and confusion.
Also, *EVERYTHING* has levels now. See that beggar sitting over there? Well, better not mess with him as he is a 50th level beggar, and he'll kick your butt! (This is a slight exaggeration, but really only a slight exaggeration.)
In general, the sessions with 3.5 have a lot more of a technical/rules heavy flavor and a lot less of the free-wheeling, adventuring feel that the original game had.
If you would like to try something closer to the orginal experience, check out my WWW page at:
www.drgames.org/tsyfindex.html
Drop me a line if interested.
In service,
Rich (Dr. Games)
www.drgames.org