Is the MDF baseboard going to cause me any problems down the road? Will it look different (finish texture/sheen) from the wood casings around it (painted white semi gloss)?

Jeff

However, making a scarf joint---the angles cut used to join longitudinal pieces of trim are very difficult to make with NDF as the sharp edges of the scarf joint break off easily and making a coped joint for inside corners has the same problem.

If you can solve or work with those gotchas, and you prime and paint the trim(Not just paint), you should have a nice looking job.

Really?

MDF dings and chips much worse than either poplar and pine in the installations I see.
If you have all inside corners you can probably get away with MDF.
If you have any outside corners it is very weak and chips out.
Even a vacuum cleaner hitting the corner can cause damage.
If you want nice molding go to a real lumber yard.
The big box places get there from second rate companies that start with 1x lumber that is down to 3/4 inch.
The baseboard is thus 5/8 thick instead of 3/4.

I referred to the linear sections---and the paper type MDF---not the pressed sawdust that is sometimes sold as MDF.