From an IT side (Networking)
1) WiFi accessibility issues (in park), load issues, and overall poor performance from the network. Yes, this IS an IT issue as the network must be designed to handle the intended traffic and cover the areas that it's intended to cover. Since it doesn't, they failed at meeting their need.
2) Internal systems have had major network connectivity issues in the past, from the system "just not working" to data not being properly stored/transferred. This is a combination of Programming and IT issues, depending on the true cause (code/hardware)
3) Web Server infrastructure is very often throwing HTTP errors (500s are common, indicating server connectivity). This is a fail on IT's side like the WiFi, they did not design a powerful enough network to handle their needs. They need better load balancing and server routing/caching hardware/software. Web server performance also falls under this. The web sites for WDW (and other Disney companies) are notoriously slow. They absolutely know they're a very high traffic/demand application, as such, the system must be designed to handle it.
Not IT, but falling under the general umbrella that many call "IT"
4) Web Application functionality is always iffy. Many display bugs (see recent ADR count and ADR time bugs for examples) that simply should never be released. This is either Development or QA fail
4b) Mobile Application functionality is also iffy. Many of the same bugs with the web development side.
5) The MDX site cannot be accessed from Canada. It's supposed to be. This is a huge development/IT fail. The fact that they've let it go on this long is a Product Management fail.
6) There's a loading screen between each page. In 2013. This signifies that the back-end code is too slow for proper loading. This should not happen this day in age (or is it "day and age"...whichever is correct, use that one

). Development (if code-based), PM (if by design), or IT (if hardware-based) fail.
7) Constant failure at loading tables for restaurants before ADRs open up. Depending on where the delay is, this may be a Restaurant Management fail or a Data Entry/IT fail.
8) Software PM (Project/Product Management) and Software Design tending to add extra unnecessary bells and whistles (loading screens, fancy buttons, etc) while removing useful features (ADR date range, streamlined ADR process, etc)
Note that: "Sometimes it's slow" can very likely (depending on where the slowness comes from) be a failure on the part of IT. As described above, it is their job to design and maintain a network that is able to keep up with the server loads the web application demands. If it's an issue in routing to their datacenter(s), that's nothing they can help, but routing and server load issues within their datacenter(s) absolutely is their job.
There's more...but this should be a start.
Note that many people use the term IT loosely to cover all Web/App/Internal Software Development and Hardware needs. I tend to be a little more specific in assigning my blame, but the entire software engineering team as a whole has a ton of issues that present themselves every day.