Some days I think the existence of fireball and wish blinds us to the simple fact that first level magic-users kick ass. Here are some brief thoughts on the eight spells any M-U in OD&D can cast.
Detect Magic - Not every magic sword will glow like a lightsaber when drawn from its scabbard. That stick could be a magic wand or a conductor's baton. Is our ally cursed or charmed (see below)? You find a pool of water in the dungeon. Care to find out it's enchanted after you wade into it?
Hold Portal - Knowing the bad guys can't get to you, even for a few turns, is golden in retreat situations. And you want to really mess some NPCs up? Lure them into a house, hold the portals and set the joint on fire. Here's another one: a pit with a hatch is just more flooring while its held.
Read Magic - Worthless scrap of paper or wish scroll? Only the magic-user knows for sure. And the OD&D version specifically mentions that this spell can be used on items. And when you think about it, this spell is the gateway to real ultimate power; all other MU spells flow from this source.
Protection from Evil - Here's an easy way to really piss off an enchanted creature: stand in a doorway with this spell cast on yourself. The creature can't get past you to your non-protected allies, who can then leisurely ready their oil flasks, scrolls, etc. Heck, with 6 turns to work with, they might even be able to take an alternate route to the creature and attack it from behind.
Light - I think almost every DM goes a lot easier on the subject of lighting than they should. I know I do. If we handled torches and lanterns in any realistic way this would be one of the most popular spells ever. You ever try lighting a lantern without matches? How about making an accurate map by torchlight in a drafty catacomb? I think those tasks are approximately one bajillion times harder than we usually run them. And even if you don't want to be a dick about lighting all the time, drop the party into a pool of water and all their normal light sources are suddenly useless.
Charm Person - In OD&D this spell lasts until someone successfully casts dispel magic on the victim. Think about that for a minute. If there is a single MU1 in the campaign world then you pretty much can't trust anyone that might fail a save vs. spells. Smart MUs will probably pick one or two choice victims in any area. Charm too many and you run the risk of a random dispel magic catching one.
Sleep - Get out any old module, the low level kind that comes with a base area like a small town. Look at some of the entries and ask yourself how many of the homes and business would be vulnerable to a single sleep spell. That master burglar plaguing the area isn't a high level thief. Dude's a pipsqueak first level magic-user who peaks in the window, throws sleep, then casually makes off with the tea service and the jewelry box.