I learnt my craft on AD&D 1st edition and my fondest RPG memories are tied up with it. This is probably due more to the nostalga of youth than it is to the quality of the game!
The main thing to bear in mind is that, outside of combat (and precious little in there), there are virtually no game mechanics for anything. That means that as DM you will need to adjudicate virtually everything. E.g. getting past the guard in 3.5 may require a bluff check - in 1st edition you would role-play the encounter and the DM would decide if the guard was convinced.
The combat mechanics are also rudimentary and they can quickly become my attack - your attack - my attack etc.
The XP system is exponential but the XP per encounter is linear - which means that everyone stalls at about 9-10th level, with each level taking about as many encounters as all the levels before it. I played and DMed in many campaigns and they all ran out of steam at about this point.