So far I've got
-Made immigration easier as a result of the Schengen agreement
-Allows more extreme parties to have more of a say in the European Parliament
-Affects internal balance of power as new members bring different ideas to the table
-Reform of EU institutions to cope with influx of new member states
1) That depends, many countries kept restrictions on immigration after enlargement and other countries, particularly France suspend the Schengen agreement as and when they see fit.
2) If you suggest that there is the possibility that more MEP seats means more chance of extremist parties having an opportunity to jump and down and shout lots in what's essentially a talking shop (EU Parliament) then maybe you're right. As for having more of a say in EU Governance forget it.
3) You're joking right? Internal balance of power is Germany and France - more Germany at the moment - particularly with the Euro crisis. All other countries are basically told to either take it or leave it.
4) I presume you're using the word reform in the loosest possible term?
Hope that helps