I think some of the biggest differences between online and offline play are as follows (anyone feel free to correct me):
1) Lag/Ping --> some have a lot of it, some dont. Some with no lag are terrible players, some with high lag are awesome players. It can be worked around, it just takes some time and practice.
2) Players --> Though UT2's bots are much better than UT1, there is still quite a difference. For one, players are vengeful, for two, players are unpredictable, and for three, players are a lot more stratified in terms of play level. Some players can move around and be in the air 90% of the time as well as tele-jumping around and dodging, some just run up with the minigun and hold down the trigger. The learning curve is steep, but remember that unlike bots, you usually wont have a game where every player is the exact same skill level nor tactics. I was online the second day after i got UT2, and though i wasnt a newbie (i had played UT1 a lot) i was getting fragged left and right. I am still not great but in a matter of a few days i learned to hold my own and how to accomidate for lag and skill enough to get the occasional kill of a skilled player.
3) Map Knowledge --> The best players know the maps they play on like the back of their hand. While id argue that the ability to learn map layouts is one of the key skills of any gamer, here it is imperitive. If you dont know where the weapons are, you get fragged. If you dont know where the enemies come from, you get fragged, if you dont know where the health is, you get fragged. Bot usually follow certain pathways and therefore can be predicted, and some do not / always go for certain powerups. Bots, to put it simply, dont learn a map, they know some of it and thats IT. Players know knooks, crannys, powerups, how to get them, how to defend them, and how to avoid them. If you know a map well and you are playing on it, you will see yourself doing much better.
Just a little example, and this isnt even online, but in the original UT the first map i found and clinged to was DM-Morpheus. Once i learned the game and started being able to hold my own in most levels, i could still play much better in morpheus. In case you wonder how much better, i averaged Adept/Inhuman for bots after a month or so on most maps, sometimes godlike, on morpheus i usually won a godlike match by 10+ kills in a 30 kill game. It helps to know your maps.
4) Finally, know your weapons and what levels will call for which ones. The flak cannon is awesome for narrow hallways since it bounces around off the walls, but is not that great in more open levels. Likewise the sniper rifle is best in hilly, wide open areas. The redeemer is best in an open or flat area, and a rapid fire gun is usually the best choice if you have the damage amp since it increases your chances of hitting the opponent.
I dont mean to sound overbearing, just some tips if you find yourself having some trouble. However, as i said, the learning curve is steep and initially not very gentle. However, if you can play against humans well, bots will crumble like tin foil before you.
Thank you for saying so!