Any notable differences between Android versions?

Are there any noteworthy differences between the different versions of android (Ice Cream, Honeycomb, Jelly Bean, KitKat, etc)?  I’m looking to buy a tablet, and I’m seeing these different versions on different tablets (offered by NewEgg).  Specifically, is there any version of android I should (or should not) get?

I’ve heard that not all tablets receive android upgrades in a timely manner (some might never get one).  How problematic is having an older version of android?

Don’t even bother with anything older than 4.2. If you buy a tablet with Honeycomb (3.x), you’re stuck in 2011. You also won’t be getting critical security updates. Anyone selling devices that old today is pretty much guaranteed to never update them and not give a damn about its existing customers.

If you want your best chance at getting the latest updates, get a Nexus 7 or a Nexus 10. Both are excellent tablets in their class and come with 4.4. Reasonable options also include a table from a big-name manufacturer (e.g. ASUS, Samsung, and the like) that at least comes with at least 4.3 and has publicly stated their intent to provide an update to 4.4 in the near future. Anything else (e.g. random Chinese tablets and rebranded versions thereof) is just likely to leave you behind.

Another plan is to get a device supported by CyanogenMOD and use that, which is then likely to get updates even when the manufacturer doesn’t.

Having an older version of the OS means you don’t get new features, bugfixes (e.g. with >=4.2 or so you can get proper Kanji fonts if you switch your system language to Japanese, and I guess they will eventually fix it in a future version so you can configure it even if your system language is set to English), security updates, and, eventually, newer apps. Also, for example, Jelly Bean is significantly faster/smoother than Ice Cream Sandwich.