I am thinking about buying a fly rod after the fun I had today with one. Which wt. would be best all around for mostly bluegill, the occasional trout, and the occasional river smallie?

Sounds like you might want a 5 wt rod and line.  very common "all arround" line.  If you are looking for more "play" then you might go down to a 3-4wt.  

Bluegills will bite anyline.  The heavier line might spook small trout but works well with smallies.  The lighter lines are harder for smallies but good for river trout.

But since you said "mostly" bluegills, my vote is an all arround 5 wt.

Vic

For what you describe I would go with a 4wt. It really all depends on what type of flies you want to be casting. The small nymph type of stuff, and little poppers you toss at gills and trout will be fine on a 4 weight. When you want to start throwing bigger heavier stuff at smallies you will want a six. Then you will want to start throwing bigger more wind resistant flies and get an 8, then you can hit the salt.

Pretty soon you will have a basement full of fly rods and be building a 10wt in the hopes that it will force you to book a trip to tarpon country. ;D

building a 10wt in the hopes that it will force you to book a trip to tarpon country. ;D

That would be awesome, but I don't think I'm ready for that yet.  ;D

I've seen those runs Tarpon make on fly tackle, incredible.  

I second the 5 wt recommendation.  It will let you get the smaller stuff, but you can still fight both largemouth and smallies.  The limitation is that you won't be able to cast some of the bigger bass bugs very far.  This is the rod that I started with.  I now have several 5 wts, a 3 wt and 7 wt.

I have 15 rods, 0 weight up to 10.  6 of them are 5 weights. Get a 5, in fact, shoot me a PM if you are interested. I have a decent beginner setup, complete with blank warranty card on the rod. Oh, it's a 5 weight, 4 piece Winston Ascent, also have a Ross reel with SA line to go along with it. Might be willing to make a swap otherwise too

Something that hasn't been said is to try and test cast anything you are thinking about buying.  Rods vary a lot and what some find a joy to cast you won't like the feel of.  Good fly shops will let you test cast rods, if they won't move on.  

I'd recommend a 4wt. to 6wt. If you mostly fish. If you fish mostly Bluegill with occasional trout, I'd go with the 4wt. Lots of fun for Bluegill. 5wt. for Bluegill, trout and an occasional smallie. 6wt. for trout, smallies and occasional Bluegill.