Well, in a (kupo)nutshell, a lot. You will have access to everything that was in the game added up to the 2.55 patch (in other words, the game as it was just prior to the expansion being released), which is still a lot of content. The only drawback is the level cap will be locked at the default level of 50, as the expansion is required to level beyond this to 60 even if you don't access anything else, not to mention the main story scenario will remain frozen on it's cliffhanger at the end of the 2.55 story (it is continued in the expansion).
So it is well worth investing in the expansion eventually, even if you don't get it right away, as there is still plenty to tide you over til then.![]()