The biggest issue is the performance difference. It takes an entire second to syntax highlight a new file and large files are unusable. Almost all interactions

The biggest issue is the performance difference. It takes an entire second to syntax highlight a new file and large files are unusable. Almost all interactions have a noticeable delay, including typing. Scrolling is painfully slow. At least on OS X, it's by far the app with the largest energy impact. It's the only other thing that competes with Chrome. Besides that, it doesn't use native UI elements where it could. Plugins don't add top menu items, the folder view on the left isn't a native treeview, it renders its own (slow) scrollbars, it doesn't respect native text rendering settings like line height or font hinting, pop-ups overlap in surprising ways (sometimes scroll bars are above them) and can't easily leave the main window, various elements move around unexpectedly when scrolling to an edge.