You mentioned putting tack in there, which never even crossed my mind... How specific should I be, exactly? Or is that dependent upon who the leaser is?
It's really up to you. I've tried very hard to make sure all of my tack is comfortable for my horse, so I wouldn't want someone pulling out some random extra-wide or extra-narrow saddle and throwing it on my horse. My first preference would be for someone to use my saddles, but if they wanted to use their own for whatever reason, I'd just want to check the fit first to avoid tack-related soreness.
Could you possibly tell me about any problems you remember arising when leasing? I'm worried that I might have some issues sharing him with someone else, like not being able to take him on long weekend trips that might interfere with the leaser's riding time and such... Having someone else to consider would be difficult for me to adjust to. I think my mind is going into hyper-drive trying to think of little things that can go wrong and problems that might arise, haha...
The only real problem I had with leasing was when I full leased a horse, and the owner allowed me to sublease to another girl at my barn. She was a REALLY timid rider, and anytime the horse would toss his head even a little, she'd back off. It was OK while I was there and riding him regularly, but then I had to leave for 2 months for work. By the time I got back, he was so used to not having to work, that when tossing his head didn't work on me he escalated until he found he could buck me off
Even though my horse now is a completely different personality, I've decided that if I end up going on an extended business trip again, my horse will be going in to at least part time training during that time!
I didn't have a trailer, and the one time my subleasee took him to a show, I entered in it too. (It was quite nice to have someone to share show expenses with!) so I never had to worry about coordinating use for trips. I think most people would be OK with swapping days here and there if you wanted to go away for a weekend occasionally, unless that person has a pretty rigid schedule and really needs a weekend day.