I'd love for the ability to compare two decklists side by side. Maybe add decklist comparison as a tool on an individual decklist page, which takes you to a page that shows the current decklist on the left, your list of decks on the right column, and when you select a deck from the right, it loads that list, then in a box underneath the two lists, shows a parsing of what's removed from the left decklist in red, and what's added to the left decklist to get to the right decklist in green.

Ex: R burn deck in the left column, RW burn deck in right column, underneath, it has a red colum with the list of removed cards, and a green column with the list of added cards.

Cool idea! Wrote it down.