Most developers agree the donation model isn’t great (giving the plugin away for free and requesting, not requiring, a small donation as a sign of support).
Arguably the best alternative is freemium model (where you have a free plugin and build premium functionality plugins around it) which is growing in popularity and is used by plugins such as WooCommerce and EDD.
Freemium may not be suitable for all plugins however, as some may not have many features which you can make premium, or you may just want to avoid the burden of supporting users who’ve purchased a premium product (who’s expectations for support may be far greater).
One of my plugins, Download Monitor, has always been donation based and free on WordPress.org. Although there is space for a few premium extensions, I’ve not had the capacity nor will to build them yet. When I rewrote the core plugin however, I did take out a feature I deemed to be bloat and made it separate – but not premium, as I thought users would react badly to a previously core feature being made paid-for after an update. Instead I made it “pay what you want”.