The expansion will certainly introduce the "Hero" classes (for characters from level 60 on) and thus access to new, above-level-60 areas (Northrend? Hyjal?) as well as new classes and maybe even new races/factions (Neutral? I would like that on PvP servers), and probably weather effects. As non-expansion player (character level limit 60), you won't be able to play in above-level-60 areas even if you have access to them, because the level 65, 70, 75 and higher monsters there will kill you quickly. I for one wouldn't hesitate a minute to buy the expansion if the new contents and options are good, because 30 to 50 bucks is really nothing when you are used to buying entertainment articles like DVD's, videos or music CD's these days ;)
PS: A tip for the heavy MC, Onyxia and BWL item farmers: Once the WOW expansion is out, and level 70,75, 80 and higher items can drop, you can forget all (epic) boss drops that are currently considered godly by non-expansion standards, because any "green" or "blue" level 75 or 80 item will outperform those easily then. So, don't waste too much time in MC raids but start new characters and enjoy the new experiences ;)
PS: A tip for the heavy MC, Onyxia and BWL item farmers: Once the WOW expansion is out, and level 70,75, 80 and higher items can drop, you can forget all (epic) boss drops that are currently considered godly by non-expansion standards, because any "green" or "blue" level 75 or 80 item will outperform those easily then. So, don't waste too much time in MC raids but start new characters and enjoy the new experiences ;)
"Man only plays when in the full meaning of the word he is a man, and he is only completely a man when he plays." -- Friedrich von Schiller