I guess its the little things that matter a lot when you're trying to make a game look professional. I would say:
-Bother to make an icon for your game
-Try not to make your game look like it is made up of frames, try to make it more seemless by adding some tranition effects which have already been mentioned.
-If you have lots of sound effects, or play lots at once, load them externally
-An options menu
-Make sure you can pause your game
-Don't use crazy or unreadable fonts, even if you think they look cool but for God's sake don't use the default font either!
-If you're going for a professional look over size then make the install program have side pannel pictures and don't force the install to create a icon which clutters up desktops.
-Make a decent readme or even an html helpfile
-include the dll files people need to play the game
-The player shouldn't be able to find a bug for at least the first fifteen minutes
-If there's an pause-menu options screen and a title-menu options screen, make sure that both of them have the same content.
-Make the user interface simple and efficient. Nothing looks more unprofessional than a cluttered UI that's difficult to navigate
-Have customizable controls w/o having to go through MMF's default control configuration screen.
-Have a consistent style throughout the game
-If you have save files, encrypt/hide them
-Don't automatically save files in C:\Windows
-Don't use 2-frame animations.
Basically, it's all about 1 thing:
Making people forget that they're playing a game.
Just like how professional movies make you forget that those tears and laughs are fake, that cameras are following them, etc. A professional game makes you forget that you're playing a game. It's not about little details like transition or full screen, it's about the big picture, on how people aren't reminded that it's a game.
Little things like bad spelling in conversations really kill the mood. Professional games have a non-intrusive interface, where you can get missions and stuff without having to move to some big, ugly black screen.
This also includes impossibly hard levels where the player just doesn't feel smooth moving from one level to the next. It shouldn't be too easy, but neither should you have to memorize the exact paths of every enemy in the game. Extroadinarily difficult games are works of amateurs .
Also, a good game shouldn't require a manual or even a readme read to play. It should explain enough within the tutorial, but that doesn't mean the manual/readme doesn't contain tips or detail on the little bits in your game. E.g. if you have an advanced RPGs with all these stats, the stats should be perfectly understandable without looking at the manual, but a look at the manual would explain the formulas and stuff just to clear any confusion.
Of course, there's also those little details outside the game, like installers and credits, but most people don't really notice those, so they're very low on the list.
Disclaimer: Any sarcasm in my posts will not be mentioned as that would ruin the purpose. It is assumed that the reader is intelligent enough to tell the difference between what is sarcasm and what is not.