Sadly, Teekay is spot on. Or, play online, yet just be a lone wolf up 'til you meet other human players who do not mind teaming up. To level quickly while lone wolfing online, consider high density higher level monster areas:
e.g.:
pitt lvl 1, 2
dur lvl 2, 3
chaos is always a good one. Even speed doing seals, bosses, super monster packs, then Diablo
Last, yet not least: there are lots of good levelling spots in act 5. Most stick with grinding WorldStone 1, 2, 3 and throne, then baal
Have fun and don't get all caught up in the bot stuff. Just have fun and enjoy the Summer.
I will agree online is still better for solo play. The trade-off is essentially /players8 for ability to mule. Due to all the bots the added benefits of online brings chanting, waypoints, easier access to rushes. You may not get the same exp gains as playing on players 8 but you can still tag along the odd decently full games (3-6 players) and do your own thing.
Also correct me if I'm wrong but I believe there are a few additionals only available online as well such as ubers, clone d, some runewords/recipes.
I play a lot of solo.. but personally not a "big fan" of single player without some kind of import/export trainer for storage as chances are you will find amazing things that are useless to you and never exactly what you need/want to get ahead. Makes for a lot different playstyle tho having to actively manage your stats due to not knowing your "mid game" to "end gear" from start to finish.