You are not in danger of being outdated for a while yet. Tiger is due out in the first half of this year but you should be fine with Panther for a good while yet.
Highlights of Tiger:
1) Performance improvements - Tiger will likely be snappier and has some incremental performance boosts. Partly optimization, partly the effects of GCC4.
2) Spotlight - Fast searching of the system built into the OS and integrated with the core apps. Other people are more excited about this than I am. I'll stick to Quicksilver. It is useful though.
3) Dashboard - Mini programs to add simple functions to the OS. Similar to Konfabulator. They are very simple to write, lowering the threshold for simple control apps.
4) Automater - never used it. Can see the speil on Apple's site though.
5) XCode 2.0 - probably the most important new feature for me. XCode 2.0 has some nice features to make development easier.
And quite a bit more. If you want to get a better idea check here
http://www.apple.com/macosx/tiger/spotlight.html
In a nutshell you won't need to upgrade for quite a while yet unless you have a need to always be on the current release. There is no "must have" features being added but the improvements and features being added are nice.