[niemela]

On Euler you're asked for the answer, not an implementation solving the problem. This means that you can solve the problem with an implementation that takes days to run or require some manual steps. On Kattis you submit source code to the system, so the runtime limit will be on the order of seconds (definitely not days).

Topcoder has "more interface". With Kattis you would typically code in whatever editor/IDE you have and prefer. Also, Topcoder asks you to implement a function, Kattis asks you to write an entire program. The most important (?) difference being that the latter includes I/O.