Spell durations that are measured in terms of 6-second combat rounds make it more difficult to use those spells outside of combat. I think it also contributes to the "need" for a wizard to have something to do every round of combat.
In AD&D, sleep lasted 5 minutes per level. In Basic D&D, it was 40 to 160 minutes. By 3.5, it was down to 1 minute per level (and the target now gets a saving throw). In 4e, the target is slowed and then falls unconscious 50% of the time, with a 50% chance of waking up after 6 seconds.
In BD&D, if you cast sleep on a couple of guards outside of combat, you'd actually have time to do something while they were sleeping. The same goes in AD&D, though you might need to gain a couple of levels. In 3.5, you'd best tie them up unless you're already 10th level. In 4e, you're not even going to have time to tie them up unless your party members physically restrain them -- assuming the spell didn't just slow them down.
Hold person is another spell that used to have a lot of non-combat uses. In BD&D, its duration was 90 minutes. AD&D had it at 4 minutes + 1 minute per level. 3.5 brought it down to 1 round/level (and also made it only affect 1 creature instead of 3 or 4). If somebody wasn't listening and kept talking over you, you could hold them so they had to stand there and listen. At 6 seconds/level, you're mainly just able to paralyze a creature so you can tie it up.
Yes, it sucks when your PC is asleep or held in combat and you can't do anything while everyone else gets to take a turn. Not to harp on it, but if combat doesn't take 45 min to an hour and can be resolved in about 10 minutes, then it's not really that big a deal to be on the sidelines.
I much prefer long-term buff spells to ones that are measured in rounds. I especially hate the 4e "+X to attack/damage/defense until the start/end of your next turn" durations -- everyone in my group finds them a nightmare to keep track of. Bull's strength in 3.5 lasts 1 minute per level, or basically long enough for a single encounter. Better than having to track what the floating bonuses are on any particular round. I think I prefer the AD&D strength spell: it lasts 1 hour per level, and it gives a random bonus to strength (1d4 for magic-users, 1d6 for clerics & thieves, 1d8 for fighters). It caps at 18 strength, but allows fighter classes to improve exceptional strength for the duration of the spell. This spell won't make the cleric a better fighter than the fighter and it lasts long enough to be useful for both exploration and combat.
I don't see a need to have a buff spell for every ability score. I think that AD&D just had strength and friends, the latter of which boosted the caster's charisma score by 2d4 points for 1 minute per level to all those in the area of effect (they get a saving throw and if they succeed, the caster gets a 1d4 penalty instead). The buff spells for all six ability scores reeks of the whole "unnecessary symmetry" aspect that seemed to drive much of 3e design. Spells that enhance your own casting stat seem like a bad idea to me. Spells that buff derived stats (attack/damage rolls, AC, saves, skills, etc) are probably a better idea than spells that buff the base ability scores.
I also miss the old style charm person, whereupon an individual of average intelligence fails his saving throw is charmed and gets a new saving throw every 3 weeks to break the spell. It was great for world-building, but also great for PCs, as you could charm people in town and keep them friendly to you for a while. 3.5's 1 hour per level disappoints me as a DM, never mind if I were playing a wizard. If a 1st-level AD&D magic-user had charm person as his single spell per day and successfully cast it on a hobgoblin, then the player gets to meaningfully contribute to the game by interacting with their new friend and possibly playing it in combat so that I don't have to do it (so long as he's not making the hobgoblin do something stupid and suicidal).
If longer-term spell effects are deemed to be unbalancing, it would make sense that a caster cannot regain use of that spell slot until the spell is ended. For example, the wizard's 1st-level spell slot is required to maintain the charm person spell over the course of days or weeks. Unless the spell is broken or he ends the spell effect willingly, he can't prepare/memorize a spell in that slot.
