Well I am guilty of auto pilot posting too often, but I'll do what I can.
I think the "problems" were not well addressed in 2e, one might as well stick to pre UA 1e(& tweak that should it be desired). Most will say AD&D breaks down past L14 or so, and they have valid views.
The wizard or MU starting weak & finishing strong is a feature, not a defect. The "balance" of AD&D is long term and more subtle than 3 or 4e. All HL spellcasters are quite mortal considering rest/memorisation rules (DMG pg 40). Spell descriptions & rules are not a strong point of 1e; changes or bannings are likely wise, although that's a huge topic on it's own merits. Some method of reigning in spell damage at extreme levels may be another good idea. MR ceases to perform this function past L20.
Some find the fighter to be a bit boring at any level. Magic items are assumed to provide interest(especially things like elemental command rings & the like), as well as provide quick combat resolution at VHL (vorpal & sharpness blades, girdles of giant strength)
A key point so many miss, especially those starting after 1e, is that level comparisons are useless at HL. The comparison is of XP totals. If your L1-20 game halts when the first characters hit L20, you get Thieves at just over 2,000,000 base XP(and +10% for 16+dex). This is compared to fighters of L16/17, clerics of L17/18, mages of L15/16. The 1e subclasses fit interestingly into this mix. A L20 MU is far " higher level" than other classes(who are in mid to high 20's by that point) & the game system is getting shaky at that point.
I hope that was of some help Irda Ranger.

