Ranked in these QuestionsQuestion Ranking
Pro Simplifies the graphic stack
Wayland simplifies the graphics stack by trying to force everything through a GEM/DRM stack and straight into the kernel. Furthermore, it manages compositing itself.
Pro Easy to maintain
Wayland has no drawing APIs. Instead, a Wayland client gets a DRM buffer handle, which is practically just a pointer to a graphics memory. Practically Wayland does not care how the client draws to that buffer, it only copies the client's buffers on the screen.
The removes a lot of complexity (because Wayland just pushes the complex stuff to the other layers of the stack) and by making the clients responsible for all the rendering, they can be smarter on how they do things like double-buffering for example.
Pro Best Linux User Experience Overall
Very responsive Touchpad performance + 3,4-finger Gestures, Smooth Animations and No Screen Tearing. Also compatibility for old applications using XWayland.
Con Little driver support
Most closed sourced drivers do not support the KMS/shared-GEM/shared-DRM technologies on which Wayland works. While this may be okay for open source purists, who only want to use graphic cards that have open source drivers available, it may not sit well with people who spend a lot of money for high-end graphic cards only to get some crappy 3D performance.
Although it should be noted that NVIDIA has declared that they will start supporting Wayland, it may take years before Wayland fully supports most high-end drivers.
Con Superior technology but still not ready for everyday use
Wayland is great for developers, it's both technologically and architecturally superior to X, but X is the de-facto standard display server protocol for the *nix world for so long that you can basically expect everything to work with X (user applications, graphics drivers, DEs, etc. etc.) which cannot be said for Wayland. Actually there are still too many issues with Wayland that I think it's still far from being ready for the general users/consumers today. It seems there are still years of work ahead before Wayland can fully replace X as suitable for everyday use other than running some GUI text-editors and IDEs for coding, and maybe by that time both Wayland and X will be replaced by something newer... Wayland surely has superior technology and design, but those don't necessarily mean much for the general users today (remember the RISC vs. CISC war back in the 90's, and that back when Linux kernel was first developed, it is arguably inferior to the MINIX kernel in terms of technological advance and architectural design)
Con No mechanisms to configure input
Tools like xinput and xmodmap that help customize keyboard and mouse input are incompatible with Wayland, have no corollary, and there is no clear roadmap for providing their functionality.
Con Not much used in the Unix world
Currently its only nearly usable in the Linux world, everything else still uses X11.