Really neat tool. I could see myself using this for a jam game when I need levels quick and the deadline is approaching, it's an easy way to add in some variety and level decor without needing to put any significant thought or time into it.
For anything larger than a jam game I think I would wind up being restricted by the limit of 8 additional rulesets - is it a technical limitation for there only being 8? Or a design choice? Either way, for a larger level with more going on I would want more than just 8.
I also had some thoughts about the object tool. As it is, it's good for placing essential level components (player, goal, doors, at most a few enemies), but I can see it becoming overbearing if you want more than a dozen level objects. The property system is great and I think it would be very useful to have a "sprite" property to make visualizing objects in the level a bit easier. An option for a grid-style object selection would work well, especially if you could set a sprite property - you'd be able to find the object you're looking for a lot quicker than scrolling through a list.
Last thing I thought of is that it would be nice to load more than one tileset at a time. When I'm making a game I organize my tilesets by theme, and if I want to make a level that mashes up 2 separate themes I'd pull from 2 different tilesets. Having the option to only load a single tileset forces me to combine all my tilesets which would make organizing and editing them a bit of a pain.
It's possible that my ideas are pushing scope beyond what you had in mind but with some additions and improvements I think this would completely replace the older and less intuitive 2d mapping tools for a lot of people. The interface is clean, easy to navigate, and it took me a whole 5 minutes to figure out how all the ruleset and pattern stuff worked.