depends if you rely on old programmes that are coded for PowerPC (PPC). If so then these programmes will not work in Lion. Examples are older versions of Mac Office, older versions of Photoshop, that kind of thing.
To check what you currently use and whether they are PPC in origin, go to:
Apple (top left hand corner)
ABOUT THIS MAC
MORE INFO
SOFTWARE (on the left hand side)
APPLICATIONS
Expand the window and look for the KIND column. Click on the KIND tab to sort the list and scroll down until you see PowerPC.
If you use any programmes that are PPC the you will need to see if there is an upgrade that it Lion compatible. Otherwise it won't work in Lion.