First, yum is the new apt, at least when it comes to using RPMs instead of DEBs. The intent of the tool is the same, but the functionality is slightly different (for instance, Debian/Ubuntu uses a separate CLI tool to query packages for contents, while yum is more of an "all in one" approach).
Coming from Debian/Ubuntu, I can easily say that you will miss the "completeness" of the available packages for download. There are several odds'n'ends that are handy to have but are not stock in a CentOS/RHEL environment. You can easily add extra repositories (the equivalent of a Debian/Ubuntu apt site) to yum, but keep in mind that you're trusting the source you're adding, before doing so. Nothing is more irritating than to find that you need tool X to do task Y, but tool X does not have an RPM available in the official repos (or worse: it depends on secondary package Z that has no RPM). Bummer.
At some point, you will be tempted to
- pull in an RPM that is not a part of your distro, or
- pull in some other package format that is not in RPM format
While this (usually) works, try to avoid it. RPM installs try hard to be distro-neutral, but I have seen malformed packages that made assumptions about certain directories being "there", where "there" is a directory path on some other distro. Generally speaking, you're better off just downloading the source RPMs and rebuilding the package; the resulting packages will be built for your environment and will not have some of the crazy pathing issues that you can occasionally see.
From your end-user's perspectives, a lot of the Ubuntu eye-candy will be gone; instead, you get a rather vanilla-looking Gnome setup, nothing wrong with that mind you, just a bit staid, that's all. The OOB software tends to also follow this "conservative" approach to a desktop as well, although adding the repo (as mentioned above) will remedy that if you need to.